In-Call Messaging

Send SMS or WhatsApp during a live voice call and feed the reply back into the conversation

In-call messaging action named send_verification_code, with a Message Before Action announcing the SMS and a Message Body containing a code variable placeholder

In-call messaging lets a voice agent send an SMS or WhatsApp message while a call is live, then continue the conversation using the customer’s reply. Use it to capture or convey information that is hard to relay by voice alone — one-time passcodes, email addresses, invoice IDs, confirmation links, or a menu of options the customer can pick from.

Twilio Required: In-call messaging relies on your connected Twilio account. Connect one under Integrations › Twilio before configuring these actions. In-call WhatsApp additionally needs a WhatsApp sender linked to the agent — see the WhatsApp sender setup section of the chat deployment guide.

In-call vs. post-call SMS

This guide covers messages the agent sends during a live call. If you want the agent to send a follow-up SMS after the call ends — for example a booking confirmation or a recap link — use the Send SMS action instead.

How it works

When the agent triggers an in-call messaging action:

Synthflow dispatches the SMS or WhatsApp through your Twilio account to the caller.
The customer can respond in one of two ways:
- Speak the answer back on the call. Works naturally for short codes like an OTP — the voice transcriber picks it up without any special handling.
- Reply by text. The reply is detected, added to the call transcript as [SMS Reply]: ... or [WhatsApp Reply]: ..., and stored in a variable the agent can reference.
The message itself can still include links or instructions, but the built-in reply path for Omnichannel Stage 2 is the customer’s SMS or WhatsApp text reply.
The agent either pauses for the reply (blocking mode) or keeps talking while it waits (non-blocking mode).

Create an in-call SMS action

Navigate to Actions in the left-hand sidebar and click Create.

Select In-Call SMS as the action type.

Enter the message body. Reference placeholders with angle brackets — for example Hi <user_name>, your verification code is <otp_code>.

Optionally set the message before action — a short sentence the agent says out loud before firing the SMS, e.g. “I’m texting the link to you now.” This keeps the caller informed while the message is being delivered.

Choose whether the agent should wait for a reply.

Wait for reply (blocking): the agent pauses until the customer replies, the timeout elapses, or the call ends.
Do not wait (non-blocking): the call continues naturally; the reply is injected into the transcript whenever it arrives.

If you enabled waiting, set the timeout (in seconds) and a fallback message for the agent to speak if the timeout fires with no reply.

Describe the trigger conditions — when the agent should send the SMS.

Click Save.

Create an in-call WhatsApp action

WhatsApp outbound messages sent mid-call require a pre-approved Twilio Content template. WhatsApp’s 24-hour session window blocks free-form messages until the customer has messaged you first, so the first message of any conversation has to be a template.

Navigate to Actions in the left-hand sidebar and click Create.

Select In-Call WhatsApp as the action type.

Paste the Content SID of your approved Twilio template. Content SIDs start with HX followed by 32 hex characters (for example HX1234567890abcdef1234567890abcdef). You can find them in Twilio under Messaging › Content Template Builder.

Fill in the template variables. Each key matches a numeric placeholder in the template body ({{1}}, {{2}}, …). Values can reference Synthflow placeholders — for example <user_name>.

Choose whether the agent should wait for a reply, set the timeout, and a fallback message, as for SMS.

Describe the trigger conditions and click Save.

Attach to an agent

Open the voice agent you want to configure and go to the Actions tab.

Attach the action under the During phase. In-call messaging is only available during a live call.

If you are using the Flow Designer, drag a Send In-Call SMS or Send In-Call WhatsApp node into your flow at the point where the agent should send the message.

Referencing the reply in your agent

When the customer replies, the reply text is stored as a variable:

Action	Variable
In-call SMS	`{sms_reply}`
In-call WhatsApp	`{whatsapp_reply}`

Reference the variable in a later prompt or action to keep the conversation natural:

The customer replied with "{sms_reply}". Confirm the details and continue.

See the Variables guide for the full variable syntax.

Best practices

Announce what you are about to do. Set message before action to something like “I’ll text that to you now” so the caller knows why there is a brief pause.
Keep SMS bodies short. Long messages are split into multiple segments and may arrive out of order.
Use non-blocking mode when the agent can keep talking. Blocking mode is best for must-have answers like an OTP.
Pick realistic timeouts. 30–60 seconds is usually enough for OTPs and simple confirmations.
Pre-approve WhatsApp templates early. Utility templates approve faster than marketing templates; approval can still take hours to days.

FAQ

Do I need my own Twilio account?

Yes. In-call messaging uses your Twilio credentials to send and receive messages. Set one up under Integrations › Twilio. In-call WhatsApp additionally needs a WhatsApp sender linked to the agent.

Why does WhatsApp need a template?

WhatsApp allows free-form outbound messages only inside a 24-hour session that the customer opens by messaging you first. Outside that window, only pre-approved templates are allowed, so the first mid-call WhatsApp message from your agent must always be a template.

What happens if the customer doesn't reply?

If wait for reply is on, the agent speaks the fallback message once the timeout fires and carries on. The variable ({sms_reply} / {whatsapp_reply}) stays empty. If wait for reply is off, the call just continues — if a reply arrives later during the same call it is still injected into the transcript.

Can the agent send more than one in-call message per call?

Yes. Attach multiple in-call messaging actions and the agent will fire each one based on its trigger conditions.

How do I use the reply in a follow-on action?

Reference it by variable. A Custom Action can include {sms_reply} in its URL, headers, or body; a prompt can include it inline. See Variables.

Does this work for inbound calls, outbound calls, or both?

Both. By default, the message is sent to the caller’s phone number ({user_phone_number}), but you can override the destination with to_phone_number in the action configuration. When provided, to_phone_number takes precedence.