Twilio: answer phone calls with a saved voice

Integrations → Twilio answers calls to your Twilio phone number in one of your saved voices. When someone calls, VoiceStudio speaks your greeting over a Twilio Media Stream and then hangs up. Synthesis runs on your computer with your chosen voice and engine. Twilio receives the 8 kHz phone audio it plays to the caller.

The same setup also runs the call agent, which places calls and holds a conversation in your voice, and can answer incoming calls instead of the greeting.

The integration is off by default. Nothing listens and nothing leaves your computer until you turn it on. It then needs a public HTTPS tunnel that you run.

What you need

• A Twilio account and a phone number that supports voice calls. Calls are billed by Twilio.
• A tunnel that gives your computer a public https:// address, such as cloudflared or ngrok. Twilio cannot reach 127.0.0.1.
• A working TTS engine in VoiceStudio. A saved voice profile is optional; without one, the engine's default voice is used.

Setup

Open Integrations → Twilio. The page is a guided checklist: each step shows To do or Done, the header shows the overall status (Not set up, Ready, Live), and Readiness in the side panel jumps to whatever is left. Each step saves on its own.

1. Twilio account. Enter your Account SID and Auth Token from the Twilio Console home page and choose Save and check. VoiceStudio checks the format; Twilio confirms the token when it signs your first call. A saved token is never shown again: use Replace or Remove.

2. Public tunnel. Pick cloudflared or ngrok. The page shows the install command for your operating system and the exact command to start the tunnel against the call gateway's port (3950 unless you set OMNIVOICE_TWILIO_PORT), not VoiceStudio's main port:

   cloudflared tunnel --url http://127.0.0.1:3950
   # or
   ngrok http 127.0.0.1:3950
   

   Paste the tunnel's https://… address into Public tunnel URL and save. Use only the origin, without a path. Check shows whether the gateway is listening; it starts when you turn on calls. If port 3950 was taken, the gateway uses the next free port and the commands update; restart the tunnel with the new command.

3. Phone number. Copy the Voice webhook URL (https://<your-tunnel>/integrations/twilio/voice). In the Twilio Console, open Phone Numbers → Manage → Active numbers, select your number, and under Voice configuration set A call comes in to Webhook, that URL, and HTTP POST. Save. The step shows Done once a call passes the signature check (rejected or busy attempts do not count).

4. Voice and behavior. Choose a voice (Default voice uses the engine's default; voices marked Can call are your verified own voice or a designed voice, the ones the call agent may use) and an engine (Active engine follows your current engine). Choose what answers incoming calls: Play greeting (write the greeting, up to 1,000 characters) or AI agent (the call agent; the greeting can then stay empty). The AI disclosure the agent opens with is editable here; without the agent, Add to greeting inserts a short one. Some places require telling callers they hear an AI voice.

5. Test. Play phone-quality preview resamples the greeting to 8 kHz, μ-law encodes and decodes it, exactly as a caller hears it. This runs entirely on your computer and pre-renders the greeting, so the first real call starts speaking immediately. When the button is unavailable, the reason is shown below it.

6. Choose Turn on calls in the header. Call your number; Recent calls shows each call's outcome.

Quick tunnels (such as cloudflared tunnel --url and free ngrok) usually get a new address on every restart. Update the Public tunnel URL and the Twilio webhook whenever the address changes. Otherwise, the signature check fails and calls are rejected.

Security model

• Separate listener. A tunnel running on your computer connects from 127.0.0.1, and VoiceStudio's main API trusts local callers as you. The telephony listener is therefore a separate server that exposes only /integrations/twilio/voice, /integrations/twilio/stream and (for calls the call agent places) /integrations/twilio/status. Every other path returns 404, including the API docs. Never point a tunnel at the main backend port, because that publishes the whole API.
• Signed webhooks. Every webhook must carry a valid X-Twilio-Signature. VoiceStudio uses your Auth Token to calculate Twilio's HMAC-SHA1 signature over the configured public URL and POST parameters. It also requires the request's AccountSid to match yours. Unsigned or wrongly signed requests get a plain 403. After 10 failures in a minute, the webhook answers 429 for the rest of that minute.
• Per-call stream tokens. Twilio does not sign the WebSocket upgrade. The webhook's TwiML therefore includes a random single-use token, valid for 60 seconds and bound to that call's CallSid. The media stream closes with code 1008 if its start message does not present the token.
• Secrets stay local. The Auth Token is encrypted in VoiceStudio's local settings store. It is never shown again, returned by the API, logged, or included in any export. Remove deletes it.
• Minimal call log. Greeting calls are kept in memory only and show the last four characters of the call ID, time, outcome and length of audio spoken. VoiceStudio does not store their caller numbers or caller audio. Calls handled by the call agent keep a local record with a transcript (see its Privacy section).
• Off means off. Turning the integration off stops the listener. Every public endpoint also checks the setting on every request.

Limits

Limit	Default	Override
Simultaneous calls	2 (more are rejected with a busy signal)	OMNIVOICE_TWILIO_MAX_CALLS (1–16)
Call length	300 s, then the call is ended (calls the agent places use their own limit, up to 30 minutes)	OMNIVOICE_TWILIO_MAX_CALL_SECONDS (30–3600)
Webhooks per minute	30 (more are rejected as busy)	OMNIVOICE_TWILIO_WEBHOOKS_PER_MINUTE
Listener port	3950, then the next free port	OMNIVOICE_TWILIO_PORT
Listener address	127.0.0.1	OMNIVOICE_TWILIO_HOST (an IP address; anything else falls back to loopback)
Greeting length	1,000 characters	—

Docker: the listener binds to 127.0.0.1 inside the container. If your tunnel runs in another container or on the host, set OMNIVOICE_TWILIO_HOST=0.0.0.0 and publish the port (for example, -p 127.0.0.1:3950:3950). Publish it only to the tunnel, never to your LAN or the internet directly.

How a call works

1. Twilio sends a signed POST to /integrations/twilio/voice. VoiceStudio replies with TwiML: <Connect><Stream> to wss://<tunnel>/integrations/twilio/stream, with the call's token as a custom parameter, followed by <Hangup/>.
2. Twilio opens the WebSocket and sends connected and start. VoiceStudio checks the token, then synthesizes the greeting sentence by sentence through the same pipeline as streaming TTS. Each sentence is resampled to 8 kHz mono, μ-law encoded and sent as 20 ms media frames (160 bytes each), so the first sentence plays while later ones are still being synthesized.
3. After the last frame, VoiceStudio sends a mark. Twilio echoes it once playback finishes. VoiceStudio then closes the stream, and Twilio follows the TwiML to <Hangup/>. A stop event (the caller hung up) ends the call early.

Synthesized audio carries VoiceStudio's usual provenance watermark when watermarking is enabled.

Troubleshooting

Recent calls shows	Meaning
Rejected: invalid signature	The Auth Token, Public tunnel URL or Twilio webhook URL do not match. This is common after a quick tunnel restarts with a new address.
Rejected: invalid stream token	The stream started too late (over 60 s) or did not come from the call that received the TwiML.
Rejected: busy	Too many simultaneous calls or webhooks. Raise the limits above if needed.
Engine unavailable / Speech failed	The selected engine cannot run. Use Play phone-quality preview to see the error.
No entry at all	The request did not reach VoiceStudio. Check that the tunnel is running, uses the exact tunnel command shown in the Public tunnel step, and that Twilio has the correct webhook URL.

Not included yet

Conversations and outbound calls are handled by the call agent. To use VoiceStudio as the voice of your own agent instead, see agentic voice. Plivo and Telnyx are not supported. Their media-stream protocols are similar, so the provider code is written to accommodate an adapter for them.