### Answer One-way audio issues with SIP trunking typically stem from RTP (media) being blocked by NAT or firewall configurations [(1) ]() . Since Telnyx confirms they're sending RTP but it's not reaching the destination, this points to a network-level issue rather than codec or SIP signaling problems . Key troubleshooting steps for your Telnyx + Retell setup: 1. Verify Transport Protocol Retell's SIP server supports TCP, UDP, and TLS transport [(2) ]() . For Telnyx specifically, TCP is recommended over UDP due to reliability factors [(3) ]() . Ensure your Telnyx trunk is configured to use TCP transport . 2. Check RTP Port Configuration Open RTP ports 10000–20000 UDP on your firewall [(1) ]() . Retell's IP ranges that need access are [(2) ]() : -

18.98.16.120/30

(all regions) -

143.223.88.0/21

(certain US traffic) -

161.115.160.0/19

(certain US traffic) 3. Verify Codec Settings Your Telnyx trunk should have

G711A

(PCMA) enabled, which you've confirmed [(3) ]() . Retell supports PCMU (G.711 µ-law) , PCMA (G.711 A-law) , and G.722 [(2) ]() . 4. Telnyx-Specific Configuration For your Telnyx Elastic SIP trunk [(3) ]() : - FQDN:

sip.retellai.com

- DNS record type:

SRV

- Number format:

+E.164

- Transport method:

TCP

(recommended) For immediate escalation: Contact Retell support at support@retellai.com with: - Call IDs from Retell dashboard - PCAP files - Telnyx analysis results - Your SIP trunk configuration details

This will allow engineering to investigate the specific RTP routing issue between Telnyx and Retell's infrastructure [(4) ]() [(5) ]() .

​ To learn more, see the sources I considered: [[1] Agent audio drops for outbound calls using SIP trunk](https://discord.com/channels/1186424376300679209/1390000275540021419) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Telnyx](https://docs.retellai.com/deploy/telnyx) [[4] Avaya](https://docs.retellai.com/deploy/avaya) [[5] Genesys](https://docs.retellai.com/deploy/genesys) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

@AI Evy We are facing a confirmed one-way audio issue with Retell + Telnyx. Telnyx has already analyzed multiple calls and confirmed: RTP from us → Telnyx: OK RTP from Telnyx → LiveKit: NOT arriving Details: Calls connect fine (SIP OK) Codec: PCMA Our recordings contain AI voice Telnyx confirmed they are sending RTP back, but it's not reaching LiveKit. This looks like an inbound RTP / media handling issue on Retell/LiveKit side. This is blocking production — could someone escalate this to engineering? We can provide PCAP + call IDs if needed. We are not using any self-hosted LiveKit or VPS. Setup is: Telnyx → Retell (direct SIP trunk) The issue is intermittent and increasing: - Some calls work perfectly - Others have no audio (one-way or no audio at all) This rules out firewall/NAT issues on our side. Telnyx confirmed RTP is sent, but not received. This looks like a media routing issue between Telnyx and Retell (possibly specific nodes or regions). Can you confirm if there are known issues or unstable media nodes?

### Answer I don't have information about known issues or unstable media nodes in the available sources. The documentation doesn't contain details about current system status, known infrastructure issues, or region-specific media routing problems. For your intermittent one-way audio issue with Telnyx, verify your Elastic SIP trunk configuration matches these Retell requirements [(1) ]() : Telnyx Inbound Settings: - Number format:

+E.164

- Codecs:

G722

,

G711U

,

G711A

- Transport method:

TCP

(recommended over UDP due to reliability factors) - Select your SIP region Critical for escalation: Since this is a production-blocking issue with intermittent RTP delivery that Telnyx has confirmed is leaving their infrastructure, you need direct engineering support. Contact support@retellai.com with [(2) ]() [(3) ]() : - Call IDs from your Retell dashboard - PCAP files showing RTP flow - Telnyx analysis results - Your complete SIP trunk configuration - Pattern details (time of day, call volume, specific regions) The engineering team can investigate whether there are routing issues between Telnyx's infrastructure and Retell's media servers that aren't documented publicly.

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

​ To learn more, see the sources I considered: [[1] Telnyx](https://docs.retellai.com/deploy/telnyx) [[2] Genesys](https://docs.retellai.com/deploy/genesys) [[3] Avaya](https://docs.retellai.com/deploy/avaya) ​