I'm trying to import a VoIP.ms number via SIP trunking

Support Help
1

Phone number: +18884092634 (toll-free)
Provider: VoIP.ms
Termination URI: sip:toronto.voip.ms
Transport: TCP
Auth: Username + Password

The UI accepts the format, but saving always returns:
“Internal Server Error”

Can you confirm:

  1. Is SIP trunking enabled on my workspace?
  2. Are toll-free numbers supported via SIP trunking?
  3. If not, can you enable it or confirm required steps?
2

Based on the available documentation, I can address your questions:

1. Toll-Free Numbers via SIP Trunking

Toll-free numbers are supported in Retell AI(1). When importing a phone number via SIP trunking, you need to provide the phone number in E.164 format, which includes the country code prefixed with a plus sign (+), followed by the number with no spaces or special characters(2)(3).

Your format +18884092634 appears correct(2).

2. Required Parameters for Import

When importing a phone number from custom telephony, you must provide(2):

  • phone_number: The number in E.164 format (e.g., "+14157774444")
  • termination_uri: The termination URI to identify your elastic SIP trunk (e.g., "someuri.pstn.twilio.com")
  • sip_trunk_auth_username: The username for authentication
  • sip_trunk_auth_password: The password for authentication

3. Transport Protocol

The outbound transport protocol can be set using the transport parameter, with valid values being “TLS”, “TCP”, and “UDP”, with “TCP” as the default(2)(4).

4. Troubleshooting the “Internal Server Error”

The documentation notes that Retell will not be able to know if the setup you provide works or not until a call is made(5). However, common issues to check include(5):

  • Verify your termination setting in your SIP trunking provider
  • Check the logs in your telephony provider
  • Ensure your username and credentials are correct

5. VoIP.ms Compatibility

While the documentation specifically mentions Twilio, Telnyx, and Vonage as examples, it states that “other telephony providers that support SIP trunks are also supported”(5).

Next Steps

Since you’re encountering an “Internal Server Error” during the save operation, I recommend:

Check out these potentially useful sources: Import Phone Number, Custom Telephony Overview

For workspace-specific settings or to enable features, you may need to:

3

Hello @cleaner

Can you remove the “sip:” from your Termination URI

4

When configuring your Termination SIP URI in Retell AI, you should include the “sip:” prefix as part of the URI format.(1)(2)

For Twilio elastic SIP trunking, the termination SIP URI you set up in Step 1 is what you’ll supply when importing the number to Retell.(1) The documentation shows that Retell’s SIP server URI is formatted as sip:sip.retellai.com.(2)

When setting up origination (for inbound calls), you specify Retell’s SIP server address as the origination SIP URI: sip:sip.retellai.com.(1)(2)

The “sip:” prefix is a standard part of SIP URI formatting and should be included in your configuration.(2)