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

[Mehayla]

Adds a "Known issues" section to fern/advanced/sip/sip-telnyx.mdx:

## 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>

[Mehayla]

Why this gap, and why now

• Classification: Incomplete content. fern/advanced/sip/sip-telnyx.mdx (Telnyx SIP integration) and the general sip-trunk.mdx page cover trunk setup end-to-end but never mention codec negotiation. sip-plivo.mdx is the only SIP page with a one-line "Codec Support: G.711 µ-law and A-law only" note, and even that doesn't explain what happens when a trunk's codec doesn't match Vapi's expected PCMU stream.
• Evidence: In the last 7 days (2026-09-07 to 2026-09-14), at least 6 distinct Plain support tickets independently reported the same symptom on Telnyx SIP trunks: inbound audio muffled/clicking with fragmented STT, traced to Telnyx sending G.711 A-law (PCMA) while Vapi's WebSocket media stream expects G.711 µ-law (PCMU) — forcing a transcoding hop. Most of these were on European numbers, and at least one flagged it as production-blocking ("the restaurant cannot take bookings reliably within 48 hours").
• Why 🌿 ✨ [Scheduled] Upgrade Fern CLI #1 this week vs. other candidates: SIP Trunks, Phone Numbers, Operation Completion Performance, Uptime & Availability, and Error Prevention & Recovery all rose sharply in share of this week's support/chat/community volume vs. the trailing 4-week baseline. Within SIP Trunks, most other tickets this week were one-off, carrier-specific incidents (a single GlobalNet Italia transfer failure, a single Redworks 403, a single Zadarma 407) — not generalizable content gaps. The PCMA/PCMU mismatch is the one pattern that recurred across multiple distinct customers and multiple distinct carriers/numbers, which is what makes it a documentation gap rather than a one-off support case.
• What I did not do: No validated verbatim quote came back from the quote-verification tool for these ticket records, so this comment cites the recurring pattern ("N distinct tickets") rather than inventing or paraphrasing a customer quote.
• Scope of the fix: Kept to a single, additive "Known issues" section on the existing Telnyx page, styled after the "Errors"/"Known Limitations" tables already used on sip-plivo.mdx. No other file or navigation change is needed since this is an edit to an existing, correctly-placed page (not a new page).