Hi, Call ID: 019d0c7f-79b9-7000-98bc-abee59b4c1ae Exact behavior: SIP signaling succeeds the call reaches Vapi successfully the assistant response is audible in the Vapi recording but on the live PSTN call, there is no audio from the assistant So this is not a 401 or timeout issue. It appears to be a media / RTP issue rather than signaling. Affected trunks: inbound call reaches Vapi successfully the no-audio issue happens on the live PSTN call after the inbound call is connected Recent configuration changes: rebuilt the BYO SIP trunk using the Telnyx EU side verified that the Telnyx number is assigned to the SIP connection Media Handling Mode is set to Proxy Accept any RTP packets is enabled RTP Auto Adjust is enabled there is no custom firewall, ACL, PBX, or IP allowlisting on my side Could you please check whether Vapi is correctly receiving and returning RTP/media in this Telnyx BYO SIP trunk setup, and whether there are any Telnyx-specific media interoperability issues? Best, Quirin

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Suggestion: Please ensure that your Telnyx SIP trunk and any upstream network devices allow bidirectional UDP traffic on ports 40000-60000 for RTP media, as required by Vapi. You can verify this by running a simple network test (e.g., using

nc

or

nmap

) from your Telnyx environment:

bash
nmap -p 40000-60000 -sU <your-media-server-ip>

If any ports in this range are blocked, RTP audio may not flow correctly even if signaling succeeds. See the Vapi SIP networking documentation for full details. Source: - [SIP networking and firewall configuration](https://docs.vapi.ai/advanced/sip/sip-networking)

I can help you @Quentin

Hey Quirin, this definitely looks like a one-way audio (RTP path) issue rather than signaling, especially since Vapi recordings have audio but PSTN doesn’t. With Telnyx EU + Proxy mode, it’s often tied to RTP routing, codec mismatch, or where media is anchored. I’d double-check codec negotiation (PCMU/PCMA) and confirm Telnyx is receiving RTP back from Vapi on the expected IP/port. Also worth verifying if symmetric RTP / NAT handling is fully respected on both sides despite “Accept any RTP” being on. Quick question: are you seeing outbound RTP packets from Vapi hitting Telnyx in their media logs, or is it silent there too? @Quentin