Formats

Use these conventions whenever a request or response field uses one of these types.

Timestamps

Timestamps use RFC 3339 / ISO 8601 in UTC, with a Z suffix and a three-digit millisecond fraction:

2025-06-06T08:53:20.000Z

Phone numbers

Phone numbers use canonical international E.164 format with a leading +, for example +972500000000.

WebSocket audio

WebSocket Sessions exchange raw mono audio in binary messages without a WAV header. The required media_format object configures each direction from the Session’s perspective:

• from_session describes audio that the platform sends from the Session to your WebSocket server.
• to_session describes audio that your WebSocket server sends to the Session.

Each direction selects one supported encoding and sample-rate combination:

You select the profile for each direction independently. The platform converts Session audio to the selected from_session format and converts audio received in the selected to_session format to the Session media rate as needed. Binary message sizes may vary. Each PCM16 message must contain complete 16-bit samples; each mu-law byte is one complete sample.

PCM16 messages can contain at most 64 KiB. A to_session mu-law message can contain at most 32 KiB so its PCM16 expansion remains within that bound.

G.711 mu-law companding changes the representation of an 8 kHz sample; it does not change the sample rate. A mu-law direction therefore uses exactly 8 kHz. A PCM16 direction can use any supported multiple of 8 kHz.

The two directions may use different profiles. For example, your server can receive pcm_s16le at 16 kHz in from_session and send mulaw at 8 kHz in to_session. See Create a WebSocket Session for the complete request schema.

The platform exchanges raw audio only. Selecting mulaw does not add Twilio Media Streams JSON envelopes, base64 payloads, or control messages.