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Hi, I have a custom SIP trunk configured with prov...
# ❓・ai-evy-support
b
@AI Evy Hi, I have a custom SIP trunk configured with provider 1Line (sip.1line.it) and I am experiencing the same issue reported by Anton in November 2024 — Retell is not sending auth after 401. My configuration: Termination URI: sip.1line.it Username: 0497986001 Transport: TCP Port: 5060 (confirmed by provider) What I have verified: Provider has whitelisted all Retell IP ranges Provider confirms they see zero connection attempts in their logs Detail logs in Retell show: "sip request timed out" Same agent works correctly with Zadarma Is the authentication bug fixed? If yes, what is the correct configuration to make custom SIP trunking work?
a
Thanks for tagging me @beatrice. I'll look through relevant sources and get back to you shortly.
Key configuration points from the documentation: The Retell SIP server URI is 
sip:sip.retellai.com
. For different transport methods, the format should be: - TCP: 
sip:sip.retellai.com;transport=tcp
- UDP: 
sip:sip.retellai.com;transport=udp
- TLS: 
sip:sip.retellai.com;transport=tls
IP blocks that should be whitelisted: - 
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) Regarding the authentication issue: The Discord thread you referenced shows a similar authentication problem where Retell wasn't sending auth after receiving a 401 Unauthorized response [(2) ]() . However, the thread doesn't show a resolution, and the user was advised to work with support . Since your provider confirms they see zero connection attempts in their logs and Retell shows "sip request timed out," this suggests the connection isn't even reaching your provider before authentication would occur. This points to a connectivity issue rather than an authentication bug. Recommended troubleshooting steps: 1. Verify the termination URI format includes the port: 
sip.1line.it:5060
2. Confirm your provider has whitelisted all three Retell IP ranges listed above [(1) ]() 3. Verify DNS resolution of 
sip.1line.it
from Retell's infrastructure 4. Check if your provider requires a specific SIP domain format in the termination URI If the issue persists after these adjustments, you may need to contact Retell support directly with SIP trace logs, as the documentation notes that Retell cannot verify if the setup works until a call is made .
​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Method 1: Elastic SIP trunking](https://discord.com/channels/1186424376300679209/1303044963616948345) ​
### Answer Based on the available documentation, when configuring custom SIP trunking with Retell AI, the termination URI format should include the port explicitly if you're using a non-standard configuration [(1) ]() . For your 1Line setup, try these adjustments to your termination URI: For TCP transport:
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sip.1line.it:5060
Or with explicit transport parameter:
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sip:sip.1line.it:5060;transport=tcp
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
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