To send appointment reminder calls at scale with Sinch Voice API v2, submit one batch request that contains a shared call flow and a separate parameter object for each appointment. You set how fast calls start with maxCps and how long queued calls may still start with ttlSeconds. Then you read each session’s result to learn whether a person confirmed, asked to reschedule, or could not be reached. Sinch’s appointment-reminder tutorial, dated September 30, 2026, labels Voice API v2 as public preview, so the design below should be checked against current documentation before production use.
How the batch request is built
A batch call reuses one call flow written in SVAML, Sinch’s call-control markup, and fills its placeholders separately for each recipient. The shared flow holds the logic: a greeting, answering-machine detection, a keypad menu, and a webhook call. Each recipient’s name, appointment time, phone number, and appointment identifier live in that recipient’s entry in parameters. The request therefore carries one flow and as many parameter objects as there are calls in the run. The overview of channels, SVAML, and webhooks is in the Voice API v2 API Overview.
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Sinch’s call-pacing guide describes the use case directly: “The Voice API v2 supports batch calling for any scenario where you dial many recipients with the same call flow, such as appointment reminders, outage notifications, outbound campaigns, or survey waves.” (Sinch Developers, Call Pacing (Batch Calls), developers.sinch.com/docs/voice-2.0/tutorials/call-pacing.)
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Four values decide how the run behaves. Three sit in the request itself, and the fourth is the key you send with the submission.
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| Setting | What it controls | Bounds stated in Sinch’s tutorial | How to choose it |
|---|---|---|---|
parameters |
Recipient-specific values, one object per call | One object per call | Match the number of appointments in the run |
batchOptions.maxCps |
Call starts per second | 1 to 1000 | Available trunk capacity and the capacity of any downstream agents |
batchOptions.ttlSeconds |
How long a queued call may still start | 1 to 10,800 seconds (three hours) | How long the reminder remains useful to the customer |
Idempotency-Key |
Prevents a retried submission from enqueuing every recipient again | Not stated | One key for the whole submission, reused only when retrying that same submission |
The bounds in the table are the ones the tutorial’s example uses. Because the feature is in preview, confirm the limits in the current API reference before you build around them.
Setting maxCps
Treat maxCps as a capacity ceiling, not a target. Start from the number of concurrent or per-second call starts your telephony trunks can absorb, then lower the figure if calls are handed to human agents. A reminder run that starts calls faster than agents can answer the follow-ups will produce a backlog of confirmations and reschedule requests, which defeats the purpose of the run.
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Setting ttlSeconds
Queued calls that have not started before the TTL expires are marked expired. Set the TTL from the reminder’s usefulness, not from the size of the list. A reminder for a 9:00 appointment that starts calling at 8:00 has a different window from one sent three days ahead. If the window closes before the list is drained, expired calls are a signal to split the run, not to extend the TTL past the point where the message is still useful.
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Always submit an Idempotency-Key with the batch. Without one, a timeout followed by a retry can enqueue the same recipients a second time. Generate the key once per submission and store it with the run so that a retry of that submission reuses it.
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Prerequisites
The tutorial’s setup requires the following:
- A Sinch account and project, with project access-key credentials.
- A voice-capable phone number.
- A Voice API v2 service. Batch calls use the project’s default Voice service unless you supply another service ID.
- Node.js 24 or newer.
- The Sinch CLI.
Build the confirmation endpoint first
The call flow needs a public HTTPS URL before the first call is placed, because the keypad menu posts the caller’s choice to that endpoint. Build it in this order:
- Expose a POST route at a public HTTPS address. A tunnel or a hosted endpoint both work for testing.
- Have the menu send a confirm or reschedule action along with the appointment identifier carried in that recipient’s parameters.
- Sign the action and appointment values so a caller or anyone else cannot forge a confirmation for a different appointment. The tutorial’s example signs them.
- Return a response immediately. Sinch’s tutorial states that the call waits up to five seconds for the webhook. A failed or late response can end the call, so queue slow work such as CRM writes or calendar updates and handle them after the response is sent.
- Write each response to the appointment record using the identifier from the call, so the result is tied to the right booking.
Design the call flow for humans and voicemail
The tutorial’s flow separates three outcomes at the start of the call. Answering-machine detection distinguishes a human or an unknown answer from a detected voicemail beep. A human hears the full message and a short keypad menu. A detected voicemail receives a shorter message with no menu, because a caller cannot respond to it in any useful way.
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- Keep the keypad menu short. Two or three choices, such as confirm, reschedule, and speak to someone, are easier to complete than a longer tree.
- Write the fallback instructions for input that times out or fails. Say what happens next, such as a repeat of the menu or a request to call back, so the caller is not left on silence.
- Speak the name and appointment time from the parameters so the caller can verify that the reminder is for them.
Submit the batch and watch its progress
Submit the batch with the shared flow, the parameter list, the batchOptions values, and the Idempotency-Key. Then monitor it in two layers. The batch summary reports queue and session states, which is enough to see whether the run is moving. It does not tell you the result for an individual person. Inspect the session or call details for that.
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| What you see | What it means | What it does not mean |
|---|---|---|
| Queued | The call has not started yet and can still be deleted from the run | Anything about the recipient |
| Expired | The call did not start before ttlSeconds ran out |
That the call failed on the network |
| Completed (batch or session) | The session ended | That a person answered. Check the session for the outcome |
Individual sessions are where you separate no answer, busy, and failure, and where you confirm whether a keypad choice was recorded. Build your reporting from the session details, then treat the batch summary as a progress bar.
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Stopping a run and handling interruptions
Deleting a batch stops the calls still queued. Calls already underway continue to completion, so a stop does not hang up people who are already on the line. If you need to change the pacing or the message, delete the batch, review the calls that were in progress, and submit a new batch containing only the recipients who still need a call. Use a new idempotency key for the new submission.
Preview status and what it means for production
The tutorial’s preview notice states that Voice API v2 is in public preview. Its stated preview terms include no service-level agreement and the possibility of breaking changes before general availability. For a reminder system, that means three things. Keep the batch-submission code isolated so an API change touches one module. Log every request and response so you can reconcile a run after a change. And plan a fallback channel for appointments that must be confirmed even if the call run is interrupted.
What the evidence does and does not show
The tutorial demonstrates the flow and reports that its author ran the core flow against real calls while writing it. That is the author’s own account, not an independent test, and the tutorial does not include a measured cohort or a comparison group. It therefore supports the claim that the design is workable, not a claim about how many customers will confirm or how many no-shows the reminders will prevent. No substantiated figure for no-show reduction was found in the official Sinch material reviewed for this article, so avoid quoting one. Measure your own confirmation rate and no-show rate before and after the change, using the session outcomes described above.
Sinch’s tutorial is dated September 30, 2026. Sinch’s documentation is the authority on current bounds, behaviour, and status, and the tutorial and call-pacing guide linked above are the places to check first.
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