Skip to main content
A stream with protocol twilio speaks the bidirectional Twilio Media Streams protocol over its WebSocket. This page lists every message and field. To connect a bot, such as a Pipecat bot, see Bring your Twilio or Pipecat app. The connection itself works as for any stream: Fonoster dials the stream’s URL and signs the request. See Session protocol: Connection and Verify request signatures.

​
Messages Fonoster sends

Every message except connected carries sequenceNumber, a string that counts up from "1", and streamSid.
​
first message
{ "event": "connected", "protocol": "Call", "version": "1.0.0" }
​
once, after connected
​
caller audio, every 20 ms
media.track is always "inbound". media.chunk is a string that counts up from "1". media.timestamp is milliseconds since the stream started, as a string. media.payload is base64 μ-law, 8000 Hz, mono, without headers.
​
caller keypress
dtmf.track is always "inbound_track". dtmf.digit is the key.
​
answer to your mark
mark.name is the name you sent. Sent when the audio queued before the mark has played, or when clear drops it.
​
last message
stop.accountSid and stop.callSid, as in start. Fonoster then closes the socket.
This is what a bot receives at the start of a call:

​
Messages Fonoster accepts

Fonoster doesn’t read streamSid on incoming messages. It ignores binary frames, invalid JSON, media without a payload, mark without a name, and any other event. The call continues.

​
Differences from Twilio

​
What isn’t supported

  • TwiML. Fonoster doesn’t fetch or run TwiML. The stream’s URL is the WebSocket itself.
  • A speech layer. Fonoster doesn’t transcribe the caller or speak text for you. The stream’s voice and speech settings don’t apply.
  • Call control. No transfer, hold or sending keypresses. Use the fonoster protocol for those.
  • Events of the fonoster protocol, such as turn events and the answering-machine verdict. Your bot gets only the messages listed above.

​
Hanging up

Closing the WebSocket hangs up the call, on every install. Fonoster can also serve Twilio’s hang-up request, where the operator turns it on:
{accountSid} is the workspace’s access key ID and {callSid} the callSid from start. The operator configures how Basic auth is checked:
  • Workspace API keys: the user is an API key ID (AP…) and the password its secret. The key must belong to the workspace in the URL.
  • One fixed pair (self-hosted installs): the user and the URL’s accountSid are the pair’s accountSid, and the password its authToken.
Error bodies have Twilio’s shape: { code, message, more_info, status }. With Pipecat, set base_url on TwilioFrameSerializer to the endpoint’s address and auto_hang_up works unchanged.