Outbound SIP audio artifacts on carrier phones (T-Mobile) — is LiveKit Phone Numbers outbound on the roadmap soon?

Telephony
1

Hi LiveKit team,

We’re building a voice AI agent using LiveKit Cloud + Telnyx SIP trunk for outbound calls. We’ve hit an audio quality issue where the agent’s voice has audible artifacts (sharp transitions, high-pitch clipping) when received on T-Mobile phones. Google Voice sounds fine.

After extensive debugging we confirmed the issue is in the SIP bridge transcoding:

  • Raw agent-side audio: clean

  • Telnyx recording: clean

  • Direct Telnyx Call Control API call (bypassing LiveKit): clean

  • Same call through LiveKit SIP bridge: artifacts on carrier phones

We tried gain reduction, LPF, soft limiter, dithering, sample rate changes, codec changes (G711U only), and Krisp. None fully resolved it. This appears related to livekit/sip#348 and we filed livekit/sip#608.

Two questions:

1. Is there a known workaround for the SIP bridge transcoding artifacts?

2. Is there a timeline for outbound calling support on LiveKit Phone Numbers? That would likely solve this for us since it bypasses the SIP bridge.

Thanks!

2

Do you have a recording of the problem? Not sure what you are talking about.