Synthflow vs Retell AI for Outbound Calling

Retell AI publishes the numbers you can plan an outbound campaign with: 20 concurrent calls by default on pay-as-you-go and $0.10 a minute to Australian destinations over Twilio, in a currency its page does not state. Synthflow gates campaigns to Enterprise and scopes concurrency in the contract. Checked 8 September 2026.

Most comparisons of these two platforms compare the platforms — channels, integrations, certifications, deployment model. We published one of those ourselves: the three-way platform comparison of Zian AI, Synthflow and Retell AI. This page deliberately does not repeat it. It compares only the machinery that runs an outbound list: the batch/campaign layer, concurrency, calling windows, retries, voicemail detection, caller-ID remediation and the identity checks that gate outbound dialling — because that is what decides whether a campaign to Australian numbers actually completes inside the hours the Australian Communications and Media Authority permits you to call.

The criteria that decide an outbound campaign, before any vendor names

Write these down before you open either vendor’s site. In this class of product the differences that matter are dull and mechanical, and they are all published.

1. Can you start without a contract? A sales-led product cannot be pilot-tested this week, whatever it does.
2. Is concurrency a published number? Concurrency is the ceiling on your dial rate. If you cannot read it off a page, you cannot size a campaign before you sign.
3. How fast can calls be started? Call attempts per second is a separate limit from concurrency and is usually the tighter one for outbound.
4. Can the calling window be expressed per weekday? Australian telemarketing hours differ on weekdays, Saturdays and Sundays, so a single daily window cannot encode them.
5. Which timezone is the window evaluated in? A window that silently defaults to a US timezone will dial Australian numbers overnight.
6. Can the window be edited after launch? If not, a mistake costs you the whole campaign.
7. Are failed recipients retried automatically? If not, retry logic is your code, not the vendor’s feature.
8. Is there a deduplication key? Re-uploading a list without one calls people twice.
9. Does the campaign expire? A list that outlives its campaign gets cancelled mid-run.
10. What happens on voicemail and on an IVR? On a cold list, most attempts reach one or the other.
11. Is there a remedy when carriers label your number? A “Spam Likely” flag destroys pickup rate faster than any script problem.
12. What is published about dialling your destination country? A rate card you can read is a different procurement conversation from a rate you must ask for.

Those twelve are the rows of the table below. They are criteria, not features, and each cell links the vendor page that establishes it.

Synthflow vs Retell AI: the outbound decision table

Every cell comes from that vendor’s own documentation or pricing page, opened raw on 8 September 2026. The phrase we could not locate a published X means exactly that: we searched the vendor’s own documentation index and the pages linked in this table, and did not find it. It is not a claim the capability is absent.

Decision criterion	Synthflow	Retell AI
Can you run a campaign without a sales contract?	No. The Campaigns documentation states: “Campaigns are available on our Enterprise plans. Reach out to our support team to activate them on your account.” The pricing page lists one tier: “Enterprise contracts start at $30,000 annually.”	Yes, as published. The pricing page lists a pay-as-you-go tier at $0.07-$0.31 per minute with “Full Platform Access”, $10 in free credits and 20 free concurrent calls; Batch Call is listed as a product. We could not locate a plan gate on the batch call documentation.
Published default concurrency	Not a number. The concurrency page gives “Scoped with Synthflow” for enterprise or account-managed deployments, and refers legacy workspaces to their account terms.	20. The concurrency page states: “Pay-As-You-Go workspaces are allocated a quota of 20 concurrent calls by default.” Burst is the lower of 3x the limit or the limit plus 300, at a $0.10/min surcharge applied to the whole call.
Rate limit on starting calls	CAPS (call attempts per second), also scoped with Synthflow. The same page advises a 1-second delay between call initiations from external automation tools to stay inside the default.	CPS, set per telephony path — separate Telnyx, Twilio and custom-telephony cards in Settings > Limits. Batch calls draw on the same pool: the concurrency page states that “a batch runs no faster than your concurrency and CPS limits allow.”
Slots reserved for inbound during a campaign	reserved_concurrency on the campaign; must be lower than the account maximum or the request fails with a 400.	Reserved concurrency in the batch dialogue; “The value must be at least 1 and at most your concurrency limit minus 1.” A batch therefore never gets the full quota.
Per-weekday calling windows	Yes. call_time_window.weekly_hours keyed by lowercase weekday, times as HH:MM. “A day you leave out means no calls that day.” Windows cannot span midnight.	Yes. The Create Batch Call schema defines windows as minutes since local midnight with an optional day list; “Cross-midnight windows are NOT allowed”.
Timezone the window is evaluated in	The agent’s own timezone. The timezone reference lists 11 Australian IANA zones, including Australia/Sydney, Australia/Perth and Australia/Eucla.	An IANA timezone field on the window object. The schema states it defaults to America/Los_Angeles when omitted — a Sydney campaign that leaves it blank runs on US Pacific time.
Editing the window after launch	No. “A campaign’s own call_time_window cannot be changed after creation.” Editing the agent’s business hours instead takes effect on the next dialling cycle.	No, once scheduled. “Once a batch is planned (scheduled for a future time), it can’t be edited.” Drafts remain editable.
Automatic retry of failed recipients	None. “Failed recipients are not retried automatically.” You re-send them in an append request.	We could not locate a published automatic retry or redial setting for batch calls (searched the documentation index at docs.retellai.com/llms.txt and the batch call and concurrency pages, 8 September 2026).
Deduplication key on re-upload	Yes. A recipient id that already exists “skips that recipient instead of calling twice, which makes retries safe. Recipients sent without an id are never deduplicated”.	We could not locate a published deduplication key in the Create Batch Call schema or the CSV column list; metadata is documented as storage only.
Campaign lifetime and list size	Seven days. “A campaign has seven days to work through its list”, after which unfinished campaigns are cancelled along with everything queued. 1 to 10,000 recipients per request, up to 1,000,000 per campaign.	We could not locate a published campaign expiry or a maximum task count in the Create Batch Call schema (no maxItems on the tasks array as published on 8 September 2026).
Voicemail handling on outbound	Leave a voicemail, as an exact message or a prompt. Constraint on the call configuration page: “Leaving a voicemail only works when the agent waits for the caller to speak first”, so the greeting must be set to Human speaks first.	Hang up or leave a message, per the voicemail and IVR page. Detection runs only for the first 3 minutes of a call, adds under 30ms of latency, and sets a voicemail_reached disconnection reason.
IVR handling on outbound	An IVR action. The IVR handling page states: “This feature is still in beta. For now, it only works for DTMF input (based on pressing digits), not vocal IVR trees.”	Two separate controls: press-digit IVR navigation, and an IVR Hangup toggle that disconnects on detection with an ivr_reached reason.
Carrier “Spam Likely” remediation	We could not locate a published verified-number or branded-calling service on Synthflow’s own documentation (searched the documentation index at docs.synthflow.ai/llms.txt and the pricing page, 8 September 2026).	Two, both documented: verified phone number and branded call. Both are “Only available for U.S. numbers at this time” and take 1-2 weeks to process.
Identity check before outbound is unlocked	We could not locate a published KYC step; onboarding is scoped with the sales team per the billing overview.	Yes. KYC verification is required “Before you can make outbound calls with Retell”, automatically, via Persona with government ID, or by manual review. “We currently support verification in 83 countries.”
Published cost to dial an Australian number	We could not locate a published per-minute rate; the pricing page and billing documentation both route to sales scoping.	$0.10 per minute for Australia over Twilio on the international calling page. Amounts are shown with a bare dollar sign and the page states no currency.
Maximum call duration	The call configuration page states: “The Max. Call Duration slider tops out at 30 minutes”; leaving the cap off removes the fixed limit.	1 hour by default, raisable to 2 hours in agent settings, per the concurrency page.

The verdict

Choose Retell AI when you need to dial a list before you can get a contract signed, when someone has to size throughput from published numbers rather than a scoping call, when the destination is an Australian number and you want a rate you can read today, or when carrier spam-labelling on US numbers is already hurting pickup. It is the only one of the two whose outbound ceiling — 20 concurrent calls, burst to the lower of 3x or plus 300 — can be planned against before you talk to anyone.

Choose Synthflow when the campaign layer itself has to be safe under repeated uploads and partial failures. Its recipient id deduplication is a published guarantee that re-sending a list will not call the same person twice, its calling windows are declared per weekday in the agent’s own timezone with 11 Australian zones listed, and its recipient states (pending, claimed, dispatched, failed, canceled) with verbatim error strings give an operations team something to reconcile against. You are buying an Enterprise agreement to get any of it.

Neither is right when the phone call is one step in a multi-channel sequence rather than the whole motion, or when you want the programme operated rather than the tooling licensed. Neither publishes an automatic retry policy, so cadence logic — call, wait, call again, hand to email — is yours to build and run in both cases. That is a build decision, not a buying decision, and it is worth being honest with yourself about which one you are making.

The seven-day campaign test: does your list fit inside Australian calling hours?

This is the arithmetic that changes the answer for Australian buyers, and neither vendor does it for you.

The Telecommunications (Telemarketing and Research Calls) Industry Standard 2017, administered by the ACMA, sets the hours in which telemarketing calls to Australian numbers may be made without consent: 9:00am to 8:00pm on weekdays, 9:00am to 5:00pm on Saturdays, no calls on Sundays and no calls on national public holidays. That is 63 callable hours in a clean week — 55 on weekdays plus 8 on the Saturday.

Now put that against Synthflow’s published campaign lifetime. A campaign has seven days to work through its list, and an unfinished one is cancelled with everything still queued. So the real question is not how many lines do I need — our published concurrency sizing method answers that per day — but will this list finish before the campaign expires.

The seven-day campaign test. Slots needed = recipients ÷ (callable hours × 3,600 ÷ average line-seconds per attempt).

Worked, with the inputs taken from our own published sizing page, where they are disclosed as illustrative assumptions rather than measured figures — substitute your own: at an assumed 25% connect rate, answered calls occupying the line 180 seconds and unanswered attempts 30 seconds of ring time, the average attempt occupies 67.5 line-seconds. Sixty-three callable hours is 226,800 seconds, so one concurrency slot completes 226,800 ÷ 67.5 = 3,360 attempts in an Australian calling week.

On Retell’s published pay-as-you-go quota of 20, with the mandatory minimum of one slot reserved for other traffic, a batch gets 19 slots. Nineteen slots × 3,360 = 63,840 attempts in that week, or about 1,013 attempts an hour. That is the ceiling on the default tier, before any burst.

Recipients in one campaign	Concurrency slots needed to finish in seven days of Australian calling hours	What that means
10,000	3	Comfortable on Retell’s default quota; trivially inside any Enterprise scoping
25,000	8	Still inside the default quota
50,000	15	Inside the default quota — 15 of the 19 slots a batch can use
63,840	19	The exact ceiling of Retell’s default 20-slot quota with 1 slot reserved
100,000	30	Requires a purchased concurrency increase on Retell; a scoping number on Synthflow
250,000	75	Contracted capacity on either platform
1,000,000	298	Synthflow’s published per-campaign maximum cannot complete inside its own seven-day life below roughly 298 slots

Two consequences fall out of the table. First, Synthflow’s headline campaign capacity of 1,000,000 recipients is not reachable within a single campaign under Australian calling hours unless the concurrency in your contract is around 298 — and the concurrency in your contract is the one thing the documentation does not publish. Second, the seven days start at creation, not at first dial: a campaign created on Friday with a Monday trigger_timestamp has already spent three of its seven days before a single number is dialled. Substitute your own connect rate and talk time; the shape of the answer does not change.

Neither platform expresses a public-holiday exception in the calling windows we could locate, so the days on the ACMA’s list — New Year’s Day, Australia Day, Good Friday, Easter Monday, Anzac Day, Christmas Day and Boxing Day — have to be handled by pausing the campaign yourself. On Synthflow that pause burns campaign life; the seven-day clock keeps running while nothing goes out.

Where each option is wrong for you

Synthflow is the wrong choice if you need to prove the concept before committing budget. There is no published self-serve path, campaigns are Enterprise-only, and the two numbers you most need for capacity planning — concurrency and CAPS — are scoped in the agreement rather than published. It is also the wrong choice if your operating pattern is many small, frequently re-cut lists: the seven-day campaign lifetime and the fact that a calling window cannot be changed after creation both favour large, well-planned runs over iterative ones.

Retell AI is the wrong choice if your safety net has to be the vendor’s rather than yours. We could not locate a published deduplication key for batch recipients, so a re-uploaded CSV is your problem to guard against, and we could not locate a published automatic retry policy, so cadence is your orchestration. Its two answer-rate remedies — verified numbers and branded calling — are documented as US-only, which means an Australian caller ID gets no benefit from either; branded calling for an Australian caller ID is a separate exercise. And outbound is gated behind KYC in 83 supported countries, so budget time for identity verification before your first dial.

Both are the wrong shape if what you actually want is the outbound programme run rather than a dialler licensed. Both sell a platform your team operates; the campaign layer is a set of primitives, and the judgement about who to call, when to stop, and what to say next remains yours.

Whichever you pick, the compliance obligation does not transfer. The Do Not Call Register washing duty and the calling-hours standard both sit with the caller, not the platform — see how ACMA enforcement applies to AI voice agents.

One thing in Retell’s own documentation we could not reconcile

Two Retell pages say different things about whether a Retell-purchased number can dial an Australian destination, and the difference is material if you are planning an Australian campaign.

The international calling page lists Australia at $0.10 per minute over Twilio and gives a worked example of a US-based team reaching customers in India, the UK and Australia using a Retell number. The outbound calling page states that Retell supports calling to 15 countries from Retell-purchased numbers. But the Create Batch Call API schema describes the destination field as: “The number you want to call, in E.164 format. If using a number purchased from Retell, only US numbers are supported as destination.” The identical sentence appears in the Create Phone Call schema.

We are not asserting which is correct — we have not tested it, and an API description can lag a capability. We are reporting that as at 8 September 2026 Retell’s own documentation states both, and that an Australian buyer should get the answer in writing before building a campaign on it. This is the ordinary case for treating documentation as evidence with a hierarchy rather than as a single source of truth, which we set out in how to weigh a vendor’s marketing page against its documentation. The safe reading, and the one that avoids the question entirely, is to import your own Australian number over SIP rather than buying one from the platform.

Methodology and disclosure

What we compared. The outbound campaign layer only: batch and campaign APIs, concurrency and rate limits, calling windows and timezones, retry and deduplication behaviour, voicemail and IVR handling on outbound calls, caller-ID remediation, identity verification, and published destination rates. We did not re-compare channels, integrations, certifications or deployment models; those are in our earlier three-way platform comparison, linked at the top.

Sources. Every Synthflow and Retell fact on this page comes from that vendor’s own site — docs.synthflow.ai, synthflow.ai/pricing, docs.retellai.com and retellai.com/pricing — opened as raw pages, not through a summariser, review site, listicle, or either vendor’s page about the other. Where a claim is in quotation marks, that exact string appears on the page linked beside it. Australian calling hours and the calling-line-identification requirement come from the ACMA-administered Do Not Call Register site, linked above.

Date. All vendor pages and the ACMA page were opened on 8 September 2026. Voice AI documentation changes weekly; re-check anything you plan to sign against.

Negative findings. Where this page says we could not locate something, we searched that vendor’s own published documentation index — docs.synthflow.ai/llms.txt and docs.retellai.com/llms.txt — plus the specific pages linked in the table. That is evidence about what is published, not evidence that a capability does not exist. Ask the vendor.

Disclosure. Zian AI published this comparison and Zian AI is one of the options discussed on it. Synthflow and Retell AI are competitors of ours. Neither vendor reviewed, sponsored or was contacted about this page. No dollar figure is attached to Zian anywhere on it, because Zian does not publish one.

Frequently asked questions

Is Synthflow or Retell AI better for outbound calling?

For a team that needs to start now and size capacity from published numbers, Retell AI: pay-as-you-go at $0.07-$0.31 per minute, a documented default of 20 concurrent calls, and a published Australian destination rate of $0.10 per minute over Twilio. For a team that needs published guarantees at the campaign layer — recipient-level deduplication, per-weekday calling windows in the agent’s own timezone, verbatim failure strings — Synthflow, at the cost of an Enterprise agreement its pricing page says starts at $30,000 annually. Both checked 8 September 2026.

What hours can an AI voice agent legally call Australian numbers?

The Telecommunications (Telemarketing and Research Calls) Industry Standard 2017 applies to all voice calls made to Australian numbers that, among other listed categories, “offer, advertise or promote goods, services, land, interests in land, business opportunities or investment opportunities”, including calls to numbers not on the Do Not Call Register. The ACMA-administered Do Not Call Register site states that “Telemarketers and researchers are only able to call during the times set out below, unless the consumer has consented to being called at that time”, and sets telemarketing hours at 9:00am to 8:00pm on weekdays and 9:00am to 5:00pm on Saturdays, with no calls allowed on Sundays or national public holidays. The same page requires that calling line identification is enabled on every call, and notes ACMA enforcement options ranging “from formal warnings to penalties of up to $250,000”.

How many calls can a Retell batch call actually make in a day?

It is bounded by concurrency, not by the batch. Retell’s documentation states that batch calls consume concurrency slots like any other outbound call and that a batch runs no faster than your concurrency and CPS limits allow, and its batch dialogue requires at least one slot be reserved for other traffic. On the published pay-as-you-go quota of 20 that leaves 19 slots. At an assumed 67.5 average line-seconds per attempt, 19 slots complete about 1,013 attempts an hour, or roughly 11,100 in an 11-hour Australian weekday window.

Do either of them retry a call that was not answered?

Not automatically, as published. Synthflow’s campaigns documentation states plainly that “Failed recipients are not retried automatically” and that you re-send them in an append request. We could not locate a published automatic retry or redial setting in Retell’s documentation index or on its batch call and concurrency pages, checked 8 September 2026. Treat multi-attempt cadence as something you orchestrate on either platform.

Can I use Synthflow or Retell AI with an Australian caller ID?

Both support importing your own numbers over SIP, which is the usual route for an Australian caller ID. Two caveats that are published. Retell’s verified phone number and branded call services, which exist to remove “Spam Likely” labels, are both documented as available for U.S. numbers only, so an Australian number gets no benefit from either. And Retell’s own API schema states that a number purchased from Retell supports only US destinations, while its international calling page lists Australia at $0.10 per minute — a conflict in its own documentation we could not resolve from published material on 8 September 2026.

Which one handles voicemail better on an outbound list?

They handle it differently rather than better or worse. Retell detects voicemail and IVR separately, runs detection for the first 3 minutes of a call, adds under 30ms of latency, and can either hang up or leave a message. Synthflow leaves a voicemail as an exact message or a generated one, but its call configuration page states that “Leaving a voicemail only works when the agent waits for the caller to speak first” — so the agent cannot open the call, which changes how your outbound script has to be written.