Hi everyone, I’ve been facing a critical audio issue between Telnyx (Programmable Voice) and VAPI. I’ve been testing different setups for over a week now, with no results. --- 🔍 Summary * I call a Telnyx number connected to VAPI. * The assistant speaks → I hear it perfectly. * I speak → my voice is severely degraded on VAPI’s side (robotic, choppy, unintelligible). * When the AI transfers the call to a human on a French landline (still via Telnyx) → call quality becomes perfect again. I reproduced this: * On two different phones (same issue), * Using the Telnyx server in San Jose (USA) to simulate a worst-case scenario — yet no audio issue during the human transfer phase. --- 📌 Technical context * I’m in Paris (France), using fiber internet, Wi-Fi Calling, and a stable mobile carrier. * VAPI’s servers are also based on the US West Coast, so the connection between Telnyx and VAPI should be solid. * These tests show the issue is not related to my phone, network, or Telnyx itself, but likely how VAPI receives incoming audio from Telnyx (Programmable Voice). --- ⚠️ Additional notes * With Telnyx SIP Trunking, the issue disappears — but: * It’s not the default or recommended setup, * And it doesn’t support call transfers, which is a dealbreaker for me. --- 🙏 What I’m looking for * A response from the VAPI team regarding this behavior. * Ideally, an official recommendation to ensure stable audio quality with working call transfers. Thanks in advance for your help — happy to share more logs or run further tests if needed.

Are your rejected or unanswered calls having issues with Vapi's identification? Mine are identified as "Silence Timed Out" or "Customer Ended Call." I now have the integration as Telnyx SIP. All of these calls are being charged to me by Vapi and Telnyx. It seems to indicate that when I have the integration with just the key, it identifies them correctly and I don't incur these charges on either side. How is your connection?

Hi Nicolas, thanks a lot for your message — really appreciate your input! On my side, I'm still in the testing phase, mainly because of a major audio quality issue between Vapi and Telnyx (using Programmable Voice). Like you, I often see calls ending as “Silence Timed Out” or similar. In my case, it’s simply because the AI **can’t understand what I’m saying**: the incoming audio on Vapi’s side is so degraded (robotic, choppy, unintelligible) that it can’t transcribe anything, and therefore doesn’t respond. Eventually, the system hangs up. Occasionally, after several attempts, a keyword does manage to get through, which allows the AI to trigger a call transfer — that’s actually what I’m trying to test. But it only works one out of two or three times, and only if I repeat the keyword five or ten times. So it’s clearly an audio input issue, not a logic problem on the AI’s side. I also tested the SIP integration (I believe you're referring to SIP Trunking), and just like you, audio quality is perfect there — no issues at all. The problem for me is that call transfers don’t work in that setup. I need to transfer calls to a landline number in France (e.g., a restaurant), and with SIP Trunking, the call instantly disconnects when the transfer is triggered — as if nothing happened. So even though SIP Trunking gives much better audio quality, the lack of working call transfers forced me to put it aside for now. That’s why I’m sticking with Programmable Voice, which handles transfers properly… but has this major issue with incoming audio to the assistant. Thanks again for your message — if you’ve got any other tips or experiences, I’d love to hear them!

I understand. I think it goes beyond the audio on your part. For some reason, my audios work in both integrations and sound fine. I spent a lot of time looking at the features of the tools. I assume that's how you're transferring them? In my case, it helped a lot to be extremely clear both in the tool and in the agent's prompt. Test it, of course, and it sounds good. I think you need Telnyx Level 2 enabled for transfers between calls in your account. Do you have it verified? But as soon as you solve that, you'll still have the problem I'm having now. By identifying these calls as "Customer Ended Calls," the operator is charging for all of them, which is a fatal error when it comes to bulk calls. My problems are simpler. I want to mask my call number with a verified one I added in Telnyx, but I'm incurring costs that I shouldn't have. With "Programmable Voice," Vapi somehow correctly identifies rejected, unanswered, or voicemail calls. This is fine because I'm not charged for these calls. However, it doesn't include the masked number, which is simply adding, for example, "from": "+Number." This is the ultimate goal. With "SIP Trunking," it doesn't identify any calls at all. Even those marked as "Customer Ended Calls" are "1" second long, and the provider charges for them. I don't even answer because they're tests on my cell phone number. Charging for unanswered calls isn't at all scalable. The only good thing is that these calls I make do arrive with the verified number I have in Telnyx. Not to mention that out of 5 calls, 2 have an error saying "An error occurred in this call: call.in-progress.error-sip-telephony-provider-failed-to-connect-call" and the other 3 that I don't answer, they charge me.

Hey Nicolas, thanks for your message and especially for the tip about needing Level 2 on Telnyx — I was only verified at Level 1, so I’ve just requested Level 2 ( https://portal.telnyx.com/#/account/my-account/verifications ) to see if it helps with the call transfer issue. That’s a really valuable lead, so thanks for pointing it out! On my side, I have to admit I recently looked into an alternative to Vapi called Retell AI, and honestly, I’m pretty impressed. What I’ve been struggling to do with Vapi for over a week — I got working in literally five minutes with Retell. It just works, it’s easier to configure, and it offers pretty much the same features — maybe even a bit better designed in terms of UI and dashboard stability. So yeah, this really cooled me down on Vapi, especially since the support team still hasn’t responded clearly to our messages. I’m seriously on the verge of switching fully to Retell AI… if I haven’t already made up my mind. As for your issue, I do understand what you’re running into, but unfortunately I can’t help much there since I only deal with inbound calls (e.g. to a restaurant), so I don’t have any problems with outbound masking or billing for test calls. That said, I really recommend giving Retell AI a try. Who knows — you might get better results there too. In my case, it was a huge time-saver and just way smoother overall. I’ll still go ahead and test if Level 2 on Telnyx solves my call transfer issue with Vapi, and if it does, I’ll update here in case it helps others in the same situation. All the best, and thanks again for sharing your insights!

@Valentin | Noodek.com I basically have been facing the same issue for several days now. My main issue is that the call transfer doesnt work for Telnyx SIP trunking. I have now looking into it a lot and I think I will switch to Twilio elastic sip trunking. I saw a youtube video that explained it and I think then this should work. Idk if this helps, here is the youtube video (its in german unfortunately: )

First of all, thank you for sharing your experience. As for me, I’m still waiting for Level 2 approval from Telnyx—we’ll see how it goes, though based on what you said, it doesn’t seem to make much of a difference.

Thank you, I will try this, but most probably will switch fully to Twilio

Hey @Shubham Bajaj , we are facing the exact same issue with Vapi + Telnyx: Caller voice is seriously degraded so assistant does not understand anything. Only happening with telnyx numbers, work well with Twilio.

@marem can you create a new support ticket and share the call_ids?

Sure, I will