Hi, I'm experiencing a one-way audio issue on outbound calls using Vapi with a Telnyx SIP trunk. Setup: Vapi assistant: Sophie (ID: 1af19de9-60fd-4e07-8f3c-6429002f6682) Phone number ID: 213ff074-2db1-4803-be92-4e50287f517b Telnyx SIP Connection: "Vapi Agent Vocaux" (ID: 2807907732806436861) Number: +33189314954 Symptom: Outbound calls only: called party hears grey noise instead of assistant voice Assistant hears the called party correctly Inbound calls work perfectly "Talk to Assistant" in dashboard works perfectly Telnyx support confirmed: RTP media FROM Vapi TO Telnyx is the issue (media origination side) Sample Call ID: 019cf71e-1697-733f-a042-a52bafb2ea0a (March 16, 2026 at 15:47 UTC) Telnyx codecs: G711U + G711A, Encrypted media: Disabled

Suggestion: Double-check that your Vapi SIP trunk credential uses Telnyx gateway IP addresses (not FQDN) in the

gateways

field. For example:

json
{
  "provider": "byo-sip-trunk",
  "name": "Telnyx Trunk",
  "gateways": [
    { "ip": "192.76.120.10", "inboundEnabled": true },
    { "ip": "64.16.250.10", "inboundEnabled": true }
  ],
  "outboundAuthenticationPlan": {
    "authUsername": "YOUR_SIP_USERNAME",
    "authPassword": "YOUR_SIP_PASSWORD",
    "sipRegisterPlan": { "realm": "sip.telnyx.com" }
  }
}

Update your credential if needed, as using FQDN can cause outbound RTP issues ([see documentation](https://docs.vapi.ai/advanced/sip/telnyx)). Source: - [Telnyx SIP integration](https://docs.vapi.ai/advanced/sip/telnyx)

My Telnyx number is configured via API Key integration (not BYO SIP trunk), so there is no gateways field. The issue is still present — outbound calls produce grey noise. Can you check on your end why RTP media is not being sent correctly for outbound calls on phone number ID: 213ff074-2db1-4803-be92-4e50287f517b ?

Got you this really looks like an outbound RTP issue where audio isn’t being sent properly from Vapi, even though the call connects fine. Since inbound and dashboard audio work, it’s likely a codec mismatch or RTP handling problem on outbound with Telnyx. Grey noise usually means Telnyx is getting packets, but they’re encoded wrong or not what it expects. I can help you trace exactly where it’s breaking and fix it. Quick one, does Telnyx show no RTP at all, or packets coming in but garbled? @fouad