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docs(sip-telnyx): document Telnyx PCMA/PCMU codec mismatch causing audio corruption #1219

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11 changes: 11 additions & 0 deletions fern/advanced/sip/sip-telnyx.mdx
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title: Telnyx SIP integration
subtitle: How to integrate SIP Telnyx to Vapi
slug: advanced/sip/telnyx
description: Connect a Telnyx SIP trunk to Vapi for inbound and outbound calls, walking through the full setup so your AI voice assistants can handle calls.
---

Integrate your Telnyx SIP trunk with Vapi to enable your AI voice assistants to handle calls efficiently. This guide walks you through the complete setup process for both inbound and outbound calls.
Expand Down Expand Up @@ -158,3 +159,13 @@ Integrate your Telnyx SIP trunk with Vapi to enable your AI voice assistants to
</Steps>

By following these steps, your Telnyx SIP trunk will be fully integrated with Vapi, allowing your AI voice assistants to manage calls effectively.

## Known issues

- **PCMA/PCMU codec mismatch can corrupt call audio.** Vapi's WebSocket media stream expects G.711 µ-law (PCMU). Some Telnyx trunks, most commonly those carrying European numbers, send inbound audio as G.711 A-law (PCMA) instead. When the codecs don't match, the resulting transcoding can muffle or clip the caller's audio, add clicking artifacts, and fragment speech-to-text transcripts.
- In the Telnyx portal, check which codec your trunk is configured to send for the affected number (Voice / SIP Trunking / your trunk / codec or media settings).
- If you can set the trunk's codec preference in Telnyx, set it to PCMU to avoid the transcoding hop entirely.
- If your number must send PCMA (for example, a European DID where PCMA is the carrier default), contact Vapi support with your affected call IDs and SIP trunk/credential ID so they can confirm whether PCMA passthrough or codec auto-negotiation can be enabled on your media stream.
<Warning>
Audio quality symptoms caused by a codec mismatch (muffled or clipped audio, clicking, broken transcripts) look similar to a noisy line, but they don't have a network-loss or jitter cause. Confirm the trunk's codec before troubleshooting network quality.
</Warning>