> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. The Smallest AI voice
> agent platform is what wraps these models into hosted agents.

# New page: SIP Wire Reference

> New deep-dive page for SIP trunk integrators: SIP Wire Reference, linked from the SIP Trunking page.

New deep-dive page for SIP trunk integrators: [SIP Wire Reference](/voice-agents/telephony/phone-numbers/sip-wire-reference), linked from the SIP Trunking page.

It documents the exact wire contract, sourced from production packet captures: inbound and outbound call-flow ladders (including the digest-auth challenge on outbound), a real sanitised INVITE with its SDP, how each of your final responses is handled (what gets retried, what gets CANCELled), transfer semantics (a second outbound INVITE carrying `X-Caller-ID`, never SIP REFER, two channels per transferred call), the timer table (RTP keepalive, ringing, max duration), and how to debug with support using the `SCL_` call ID that rides on the `From` tag of every INVITE we send.

The SIP Trunking troubleshooting section now also tells you to include that call ID in support tickets: every call that reaches us has a full capture on our side, and the ID is how support pulls it.