VAPI•3mo ago

URGENT: Caller Audio Issue When Using Telnyx

We are using a Telnyx number that we imported and it's giving a poor caller voice so the STT is struggling to interpret. It was ok at first. The assistant is fine with our Twilio numbers but we really wish to use Telnyx due to cost and "Quality". Here are a couple of call id's.. Please advise.

CC: @AJH

https://dashboard.vapi.ai/calls/0199d226-0f42-7ffb-8520-03fc4b44fe04

https://dashboard.vapi.ai/calls/0199d21c-665d-7dde-92a8-c9cd62f545aa

VAPI Dashboard - Voice AI Management Platform

Build and manage voice AI agents that conduct human-like conversations. Access real-time analytics, configure assistants, and monitor your AI voice applications.

VAPI Dashboard - Voice AI Management Platform

Build and manage voice AI agents that conduct human-like conversations. Access real-time analytics, configure assistants, and monitor your AI voice applications.

VapiAPP•10/11/25, 7:39 AM

Hi there,

Thank you for your message. Our team is currently out of the office. We operate Monday through Friday, from 9:00 AM to 8:00 PM Pacific Standard Time (PST).

We’ll get back to you as soon as possible during our normal business hours.

If your message is urgent, please mark it accordingly or include “URGENT” in the subject line, and we’ll do our best to respond promptly.

Warm regards,
Vapi
Customer Support Team

Ddistinguished-brown We are using a Telnyx number that we imported and it's giving a poor caller voic...

exclusive-coral•10/11/25, 8:42 AM

I can help you figure out why the audio quality dropped on your Telnyx number and get it back to normal for clearer STT performance. This often happens due to codec settings, media routing, or how Telnyx handles audio packets. Could you share whether your setup uses SIP or WebRTC?
@Ade

Ddistinguished-brown We are using a Telnyx number that we imported and it's giving a poor caller voic...

exclusive-coral•10/11/25, 8:50 AM

Eexclusive-coral I can help you figure out why the audio quality dropped on your Telnyx number an...

distinguished-brownOP•10/11/25, 1:04 PM

@LordXcel Great, thanks. I set up using Telnyx number import using the Vapi>Create New Number>Import Telnyx Number. This method creates a Programmable Voice API Application in Telnyx. The only option I see for setting the codec's is for inbound calls. We are inbound and outbound and this problem is on the outbound for sure, I need to test inbound. Thoughts?

distinguished-brownOP•10/11/25, 1:28 PM

@LordXcel I just tested inbound and it works perfectly.

distinguished-brownOP•10/11/25, 3:13 PM

Update: I switched from Programable Voice Integration and created a BYO SIP Trunk instead and it works perfectly. There must be an issue with Vapi/Telnyx API integration!!

Ddistinguished-brown Update: I switched from Programable Voice Integration and created a BYO SIP Trun...

exclusive-coral•10/11/25, 3:20 PM

Glad you isolated the issue, that definitely points to how the Telnyx Programmable Voice API is handling outbound media through Vapi. I can help you dig into the SIP signaling and audio path to pinpoint what’s breaking in the API integration and get it stable again. Let’s take this to privately so we can go over your setup details and logs more closely.

distinguished-brownOP•10/13/25, 4:35 PM

@Shubham Bajaj What's your thoughts on the API?

specific-silver•10/13/25, 9:56 PM

@Ade are your calls in the US?

conventional-black•10/24/25, 8:41 AM

Hi @Velcel, I need a solution for the same: Telnyx recordings provide clear voice, but VAPI's same audio is so poor that the transcriber doesn't understand it, and call is ended. Is API codec still wrong, should SIP username /password be used to connect to Telnyx?

VapiAPP•10/28/25, 5:21 AM

Are you using Telnyx for inbound or outbound calling?

Cconventional-black Hi @Velcel, I need a solution for the same: Telnyx recordings provide clear voic...

exclusive-coral•10/28/25, 11:10 AM

Hey, I’ve worked on a few Vapi + Telnyx setups and faced similar audio quality issues before. I can help you diagnose whether it’s a codec mismatch or a SIP config issue — often it’s about how Vapi handles media negotiation. Could you share which codec you currently have set in your Vapi project (e.g., PCMU, Opus, G722)? @Balint

specific-silver•10/28/25, 11:13 AM

@Balint Pay attention to the following: when using Telnyx, you can "Pin Down" you media services geography, basically, asking Telnyx to make sure that your calls are handled in a specific area. As I receall, the LiveKit servers that service Retell are located in the West Coast, so try to ping that. In addition, change the codec settings on Telnyx to use G722. The default for telephony is G711, which is an 8Khz codec, while G722 is a 16Khz codec (unlike Opus which is a 24Khz codec). 16KHz is ample spectrum for providing high-definition audio, which the AI transcriber loves.

specific-silver•10/28/25, 11:14 AM

For example, in Cloudonix, this is the reason we always process all internal audio using G722 then transcode to the world as required. When we connect to AI's, we always lock the codecs to G722, following an internal re-sampling and up-scaling, to ensure audio is perfect.

conventional-black•10/28/25, 1:21 PM

I could successfully connect Telnyx SIP, now VAPI hears clear user voice. The SIP issue was caused by that I wanted to enable Inbound as well, despite your guide showing Inbound : false https://docs.vapi.ai/advanced/sip/telnyx

How to really handle Inbound then?

Telnyx SIP integration | Vapi

How to integrate SIP Telnyx to Vapi

specific-silver•10/29/25, 8:51 PM

@Balint As long as you use full E.164 numbers - VAPI will accept your calls regardless, the inbound flag is pointless in that case.

VapiAPP•10/30/25, 6:29 AM

For inbound, you have to create a separate gateway where outboundEnabled: false and protocol is UDP. It should work as long as its in the same trunk as the outbound gateway.

URGENT: Caller Audio Issue When Using Telnyx

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