SIP Trunk Setup for Retell AI

Use Voyced as an external SIP provider

This guide explains how to configure a Voyced SIP connection in Retell AI using your existing telephone number and SIP account details.

Retell AI supports external SIP trunks with username/password authentication for outbound calls.

For inbound calls, Retell currently uses direct SIP INVITE routing rather than SIP registration. This means a full inbound and outbound connection using Voyced's standard registration-based method is not currently confirmed as supported. If you set this up and it works, please let us know.

What You’ll Need

Before you begin, make sure you have:

• Access to your Retell AI dashboard

• An active Retell AI account

• At least one configured AI voice agent

• An active Voyced telephone number

• Your Voyced SIP details:

  • Account / telephone number: Your-Voyced-DID

  • Service password: YourServicePassword

  • SIP server: sip.voyced.eu

Requirements Before You Begin

• Your Voyced account must be active.

• Your account must have sufficient calling credit and simultaneous call capacity.

• Outbound calling must be enabled in Retell AI.

• Your Retell account must have sufficient credits.

• SIP authentication uses the Voyced username and password.

• SIP signalling to Voyced uses UDP port 5060.

• Telephone numbers should use full international format.

• Recommended codecs:

  • PCMA / G.711 A-law

  • PCMU / G.711 μ-law

Retell supports outbound SIP digest authentication, but its standard SIP trunk integration does not perform SIP REGISTER.

If your Voyced service requires a successful SIP registration before accepting outbound calls, the direct Retell connection will not work.

SIP Trunk Setup Steps

1. Log in to Retell AI

1. Open https://dashboard.retellai.com

2. Log in using your account credentials.

3. Navigate to Phone Numbers.

4. Click the + button.

5. Select Connect to your number via SIP trunking.

This opens the external SIP trunk configuration.

2. Enter Your Voyced SIP Details

Enter the following settings:

The Termination URI identifies the server Retell contacts when placing outbound calls.

Replace Your-Voyced-DID with your actual international telephone number.

For the Phone Number field, use E.164 format with a leading +, for example +442012345678.

Do not enter the placeholder text as a real telephone number.

3. Configure Codec Settings

Open Additional settings in the SIP trunk configuration.

Enable:

1. PCMA / G.711 A-law

2. PCMU / G.711 μ-law

Disable G.722 unless your Voyced service supports this codec for the required connection.

Retell uses 20 ms RTP audio packets by default.

Save the codec selection.

4. Test the SIP Connection

Before saving the telephone number:

1. Click Test SIP Connection.

2. Review the result.

3. Confirm the server responds successfully.

Retell performs a SIP OPTIONS request to the configured server.

A successful test confirms that the server accepted the OPTIONS request.

It does not confirm:

• SIP registration

• Outbound calling permission

• Inbound call routing

• Two-way audio

If the test fails, review the SIP response before changing any settings.

5. Save the Voyced Telephone Number

After completing the SIP connection test:

1. Review the telephone number.

2. Confirm the Voyced server is correct.

3. Confirm the authentication details.

4. Click Save.

The number should now appear under Phone Numbers.

This adds the number to Retell's configuration. It does not register Retell as a SIP client with Voyced.

6. Assign Your AI Voice Agent

1. Open Phone Numbers.

2. Select your imported Voyced number.

3. Locate the agent assignment settings.

4. Select your configured outbound AI voice agent.

5. Save your changes.

Example:

Ensure your AI agent is published and ready to handle telephone conversations.

7. Configure Outbound Calling

Retell supports placing outbound calls through a configured SIP trunk.

Once the Voyced connection details have been saved:

1. Open your Retell dashboard.

2. Select your AI voice agent.

3. Choose the outbound calling option.

4. Select your imported Voyced telephone number.

5. Enter the destination telephone number.

6. Start a test call.

Use full international number format.

Example:

442012345678

Retell will attempt to establish the call through:

Retell AI → Voyced SIP Server → Destination Telephone

If Voyced challenges the SIP INVITE for authentication, Retell should respond using the configured username and password.

Successful calling depends on Voyced accepting authenticated SIP INVITE requests without requiring prior registration.

This behaviour must be verified through a live test.

8. Configure Inbound Calling

Retell normally receives incoming telephone calls through an externally configured SIP trunk.

Its standard inbound method requires the telephone provider to deliver SIP INVITE requests to Retell's SIP server.

Retell does not currently support the SIP REGISTER method used by conventional SIP telephones and many IP PBX systems.

Consequently, entering your Voyced username and password in Retell does not establish the registration needed for ordinary inbound delivery.

Under the standard Voyced connection method, there are no additional inbound settings in Retell that resolve this limitation.

Do not use SIP URI forwarding as a substitute.

A full inbound connection would require a supported registration-capable arrangement, which is outside this standard manual.

Testing Your Setup

Outbound Test

1. Select your configured Retell AI agent.

2. Select your imported Voyced number.

3. Place a call to a valid external telephone number.

4. Check whether the call connects.

5. Verify two-way audio.

6. Confirm the correct caller ID appears.

If the call fails, review the SIP response and authentication details.

Inbound Test

Inbound calling through the normal Voyced SIP registration method is not available directly in Retell's documented SIP trunk configuration.

A successful outbound call does not establish inbound compatibility.

Troubleshooting

SIP Connection Test Fails

Check:

• SIP server is sip.voyced.eu

• SIP port is 5060

• Transport is UDP

• Username is Your-Voyced-DID

• Password is YourServicePassword

Check the actual SIP response code.

401 or 407 Error

These responses indicate SIP authentication is required.

An initial authentication challenge is normal.

If authentication repeatedly fails, check:

• SIP username

• Service password

• Authentication response

• Authentication realm

403 Error

Check:

• Voyced account status

• Outbound calling permission

• SIP authentication

• Caller ID authorisation

• Account restrictions

Voyced does not normally require IP whitelisting.

404 Error

Check:

• Destination telephone number

• International number format

• SIP Request-URI

• Dialling permissions

• Which server returned the error

One-Way Audio

Check:

• RTP negotiation

• Audio codecs

• SDP addresses

• Network connectivity

• Any firewall restrictions on the Retell side

Outbound Calls Fail Despite Correct Credentials

Retell's SIP trunk implementation does not perform conventional SIP registration.

If Voyced requires SIP REGISTER before allowing outbound calls, authentication credentials alone will not resolve the problem.

Inbound Calls Do Not Arrive

Confirm whether the external platform supports SIP REGISTER.

Retell currently does not support this method.

Changing the telephone number assignment or AI agent will not resolve a missing SIP registration.

Final Tips

• Always use your Voyced SIP account details.

• Keep the SIP server set to sip.voyced.eu.

• Use Your-Voyced-DID as the SIP username.

• Use YourServicePassword as the SIP password.

• Voyced does not normally require IP whitelisting.

• Do not configure SIP URI forwarding.

• Never run scripts or make configuration changes on Voyced servers.

• Test outbound authentication before using the connection.

• Do not assume inbound calls work because outbound calls succeed.

Retell AI offers external SIP trunking, but its current lack of SIP REGISTER support prevents us from confirming a complete direct connection using Voyced's standard registration-based method.

Last updated: 27/10/2026

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Given the fast evolution of the telecom sector, Voyced does not offer any guarantee about the accuracy and completeness of the information offered. 
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