VoIP.ms SIP → Retell inbound calls going to voicemail (no ringing)

Support Help
1

Hi Retell team,

I’m setting up inbound calls via VoIP.ms (SIP) routed into Retell AI, and I’m running into an inbound routing issue.

Current setup:

  • Phone number is hosted on VoIP.ms

  • Calls are forwarded via SIP to Retell

  • Retell agent is created and published (Conversation Flow)

  • Goal: inbound calls → AI agent answers → optional transfer to owner

Current behavior:

  • Calls do not ring

  • Calls immediately go to voicemail

  • AI agent never answers

What I’m unclear on:

  1. Do I need to enable or request SIP inbound permissions on my Retell workspace?

  2. Which agent type should be selected as the Inbound Call Agent for SIP-based calls?

  3. Is there a specific SIP URI / endpoint that VoIP.ms should be sending INVITEs to?

  4. Is voicemail expected if no inbound agent is assigned, or does this indicate a SIP config issue?

I’m happy to provide VoIP.ms SIP settings or call logs if helpful.
Thanks — just want to make sure SIP inbound routing is configured correctly.

2

Here’s what you need to configure for VoIP.ms SIP inbound calls to Retell AI:

SIP Server Configuration

VoIP.ms needs to forward calls to Retell’s SIP server URI: sip:sip.retellai.com(1)

For TCP transport (recommended), use: sip:sip.retellai.com;transport=tcp(1)

For UDP transport, use: sip:sip.retellai.com;transport=udp(1)

For TLS transport, use: sip:sip.retellai.com;transport=tls(1)

IP Whitelisting

Configure VoIP.ms to whitelist Retell’s traffic from IP block: 18.98.16.120/30 (all regions)(1)

Additional US traffic may come from: 143.223.88.0/21 and 161.115.160.0/19(1)

Number Import & Agent Assignment

After configuring your SIP trunk in VoIP.ms, you must import your phone number into Retell AI(2)

During import, provide the termination URI that identifies your elastic SIP trunk(2)

Bind the imported number to a specific AI agent within Retell AI to handle inbound calls(2)

Without an assigned inbound agent, calls will fail—this explains why your calls immediately go to voicemail(2)

Troubleshooting Your Setup

Check VoIP.ms origination settings in your SIP trunking provider(1)

Review logs in your telephony provider and consider opening a ticket with them(1)

Verify authentication is configured (IP whitelisting or credential-based)(2)

Ensure your Conversation Flow agent is properly published and assigned to the imported number(2)

The immediate voicemail behavior suggests either the SIP routing isn’t reaching Retell’s server, or no inbound agent is assigned to handle the call. Start by confirming VoIP.ms is sending INVITEs to the correct Retell SIP URI with proper transport protocol, then verify your number is imported in Retell with an agent assigned.

4

“Hi team — I’m trying to import a VoIP.ms number via SIP trunking.

When I enter a valid E.164 number (+18884092634) and a clean termination URI (no user@), the UI throws an Internal Server Error on save.

Can you confirm whether Custom Telephony / SIP Trunk Import is enabled on my workspace, or if this feature needs to be activated manually?”