I have built several voice agents from Retell + Twilio combination. The voice agents work perfectly fine, answer the calls they are supposed to and make appointments.
The problem is the call quality. The voice keeps breaking up, much like having mini disconnections. It is not unbearable but certainly reduces the overall improvement. I do not know if it is because Twilio SIP trunking causing an issue because when I test the agent on Retell’s own platform it works fine.
Has anyone faced with the similar problem and how to fix it?
Based on the sources, here are several configuration points to check for your Twilio SIP trunk setup that could be causing call quality issues:
Check Your Termination SIP URI
Ensure you’re using a localized termination URI that’s near your region(1). Remove any spaces in your Termination SIP URI if present(1). You can expand and view your localized URIs in the Twilio console(1).
Verify Transport Method
Retell’s SIP server supports TCP (Recommended), UDP, and TLS as transport methods(2). For inbound calls with different transport methods, you need to append the transport method to the SIP server URL(2):
Since Retell’s SIP server does not have a static IP, you need to whitelist all IP addresses in the range if using IP authentication(1). The IP blocks for traffic are: 18.98.16.120/30 (All regions), 143.223.88.0/21 (certain United States traffic), 161.115.160.0/19 (certain United States traffic)(2).
Alternatively, use credential-based authentication with username and password(1).
Origination Setup
Verify your origination (inbound) is correctly configured with Retell’s SIP server address: sip:sip.retellai.com(1).
Media Encryption
Retell supports SRTP for media encryption, but transport should be set to TLS to use SRTP(2).
Since your agents work fine on Retell’s platform but experience issues through Twilio SIP trunking, the problem likely lies in the SIP trunk configuration rather than the agent itself. Double-check these settings in your Twilio elastic SIP trunk setup.
it happens everytime. The voice from the agent is not fluent. It has this ‘mini-disconnects’ as if you are talking with someone when they are in the mountains where the connection isn’t that great. And this is happening with all of the agents I’ve built.
As I said, it works fine within Retell test platform but this is happening if I actually try to call the number. Which makes me think there is a problem between Twilio and Retell connection. I have followed the guideline Retell published itself to the letter (Twilio - Retell AI). So setup settings are correct.