Twilio TwiML Stream Deep Dive: Bidirectional Media for AI Voice in 2026

Twilio Media Streams started life in 2019 as a one-way stream-out feature. Bidirectional went GA in 2023, and as of 2026 it is the substrate underneath ConversationRelay and probably 80% of every Twilio-fronted AI voice product. The format is simple, the constraints are real, and once you understand Mark and Clear events, barge-in becomes a one-line change.

Background

Twilio Programmable Voice lets you control calls with TwiML, an XML markup with verbs like , , , . The noun inside opens a WebSocket from Twilio to your server. Audio flows in both directions: media events carry base64-encoded mulaw 8 kHz 8-bit payloads (160 bytes per 20 ms frame), and your server can send the same format back to be played to the caller.

<Start><Stream> is the older one-way variant; <Connect><Stream> is bidirectional and blocks subsequent TwiML until the WebSocket disconnects. The bidirectional version added Mark and Clear events: Mark lets you tag a position in your sent audio buffer and get a confirmation when Twilio plays past it; Clear empties Twilio's outbound buffer for instant interruption when the caller starts speaking.

The 8 kHz mulaw default is the friction point. OpenAI Realtime accepts G.711 directly, so for many builders Twilio's native format is fine end-to-end. For better quality you transcode upstream to 16 kHz L16 or Opus.

Architecture

graph LR
 A[Caller PSTN] --> B[Twilio Voice]
 B -->|mulaw 8k 20ms frames| C[Your WebSocket Server]
 C -->|JSON media events| D[Audio Decoder]
 D -->|L16 16k| E[OpenAI Realtime]
 E -->|Opus or PCM back| F[Audio Encoder]
 F -->|mulaw 8k frames| C
 C -->|JSON media + mark + clear| B
 B --> A

<Response>
 <Connect>
 <Stream url="wss://bridge.callsphere.ai/realtime"
 track="inbound_track"
 statusCallback="https://callsphere.ai/api/twilio/stream-status">
 <Parameter name="tenant" value="abc123"/>
 <Parameter name="agent" value="healthcare-intake"/>
 </Stream>
 </Connect>
</Response>

// Outbound media event from your server to Twilio (base64 mulaw)
{"event":"media","streamSid":"MZxx","media":{"payload":"PT4+Pj4..."}}
// Mark to track playback position
{"event":"mark","streamSid":"MZxx","mark":{"name":"utterance-42-end"}}
// Clear to interrupt currently buffered audio (barge-in)
{"event":"clear","streamSid":"MZxx"}

CallSphere implementation

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Build steps

1. Allocate a TwiML endpoint that returns the response with your WebSocket URL.
2. Build the WebSocket handler: accept connection, parse start event for streamSid and parameters, then loop on media events.
3. Decode mulaw 8 kHz to L16 16 kHz before sending to OpenAI Realtime; Twilio frames are 160 bytes of mulaw = 20 ms = 160 samples after expansion, upsample to 320 samples L16.
4. Encode model output back to mulaw 8 kHz; chunk into 20 ms frames; send as media events with the streamSid.
5. Send Mark events at sentence boundaries; OpenAI sends response.audio.delta events that you align with marks.
6. On speech_started, send Clear event immediately to flush Twilio's outbound buffer for natural interruption.
7. Monitor statusCallback for stream-failed and stream-stopped to clean up server-side state.

Pitfalls

• One per call. Cannot fork to two AI services; must demux server-side.
• DTMF inbound only (caller-to-server). Cannot send DTMF outbound from server through .
• Mulaw payload base64-encoded in JSON; if you forget to base64-decode, you stream garbage and the model says "Hello, hello, are you there?" forever.
• Clear events take ~50 ms to take effect; do not assume instant flush.
• Bidirectional streams have a 30-second idle timeout; send keepalive media frames or expect disconnects.

FAQ

Should I use ConversationRelay instead of Streams for AI? ConversationRelay packages STT, LLM, TTS into one TwiML verb. Less control, faster build. wins when you need custom STT/LLM/TTS, multi-modal, or non-OpenAI vendors.

What is the latency of a Twilio bidirectional Stream? 20-60 ms for the Twilio leg, plus your server hop, plus the model. End-to-end voice-to-voice 600-900 ms is typical with OpenAI Realtime.

Is mulaw lossy enough to hurt ASR? For Whisper and Deepgram on names and digits, yes; ~3-5% absolute WER hit vs G.722 wideband. Transcode upstream if your trunk supports it.

Can I record a Stream call? Yes via Twilio's separate recording API; the Stream itself does not store audio.

Mark vs Clear: when do I use which? Mark for tracking playback progress (used to align tool calls with what the user already heard). Clear for barge-in interruption.

Sources

• Twilio Media Streams Overview
• Twilio TwiML Stream verb reference
• Bi-directional Streaming changelog
• WebSocket Messages reference

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