SIP Trunk Setup for Voyced – Synthflow AI

Connect your Voyced telephone number to Synthflow AI using a SIP trunk.

Synthflow lets you build voice agents that can answer incoming calls, speak with callers and carry out tasks such as collecting information or arranging appointments.

This guide explains how to connect an existing Voyced SIP account to Synthflow using its custom telephone number feature.

What You'll Need

Before starting, make sure you have:

• An active Synthflow Enterprise account with SIP trunking enabled.

• Access to your Synthflow workspace.

• A configured Synthflow voice agent.

• An active Voyced telephone number.

• Your Voyced SIP username and Service Password.

• Permission to configure the telephone connection and call routing.

Voyced SIP details

Replace the example details with your own credentials.

You can find your DID under Cloud PBX / IP Centrex > Phone Lines in your Voyced Hosted IPPBX. Select the wrench icon beside your number.

Your Service Password is available under My Profile > Settings.

The Service Password is different from your Voyced website login password.

Requirements Before You Begin

Check your Synthflow subscription

SIP trunking is an Enterprise feature.

Make sure your workspace allows you to import custom telephone numbers and configure SIP registration.

If you see an Upgrade button instead of the custom number option, contact Synthflow about enabling SIP trunking.

Check your voice agent

Before connecting your telephone number:

1. Log in to Synthflow.

2. Open Agents.

3. Create or select your voice agent.

4. Configure its greeting and conversation instructions.

5. Test the agent using Synthflow's available testing tools.

You can connect the telephone number once the agent is ready.

Check your intended call usage

Voyced provides carrier-quality service for normal retail and conversational telephone traffic.

Voyced does not permit:

• Short-burst calling traffic.

• Predictive dialling.

• Power dialling.

• Automated bulk dialling.

• Autodialler campaigns.

Synthflow has additional automation and outbound calling features, but their availability does not mean these types of calls are permitted through Voyced.

If you plan to use an AI agent to answer incoming customer calls, confirm that your intended use meets your Voyced service conditions before putting it into production.

Check the SIP connection

Synthflow supports SIP connections using UDP, TCP and TLS.

For the standard Voyced configuration, use:

• SIP server: sip.voyced.eu

• Transport: UDP

• Port: 5060

Synthflow also supports SIP REGISTER authentication for custom telephone numbers.

This is the registration method required for the standard Voyced account setup.

SIP Trunk Setup

Step 1: Log in to Synthflow

1. Open your Synthflow workspace.

2. Log in using your account credentials.

3. Open Phone Numbers.

4. Select New Phone Number.

5. Choose Import a Custom Number.

The custom telephone number configuration opens.

Step 2: Add your Voyced telephone number

Enter:

Synthflow uses E.164 format for the telephone number you import.

For example, a UK number may appear as:

+442035001234

This identifies the telephone number inside Synthflow.

Your SIP Username and Auth Username must still match the actual credentials assigned by Voyced.

Do not replace your SIP username with the displayed E.164 number unless they are identical.

Step 3: Enable SIP registration

Find:

Enable outbound SIP registration

Enable this option.

This tells Synthflow to send SIP REGISTER requests to Voyced using your account credentials.

Configure:

Synthflow supports registration expiry values between 60 and 3600 seconds.

The default value of 300 seconds is a suitable starting point.

Leave Outbound Proxy empty unless the connection requires a specific proxy or port.

Step 4: Check the Synthflow SIP address

The import form also displays an Origination URI.

This is the SIP address used when your telephone provider or PBX sends incoming calls to Synthflow.

The address depends on your Synthflow workspace region.

Keep the Origination URI supplied by your own workspace.

Do not replace the Voyced SIP Domain with the Synthflow server.

These addresses serve different purposes:

• sip.voyced.eu is the Voyced server that Synthflow contacts for registration and outbound calls.

• The Synthflow Origination URI is the destination for calls routed into your Synthflow agent.

For a registration-based setup, test whether your Voyced account delivers incoming calls to the registered Synthflow connection.

If registration succeeds but incoming calls are not delivered, the incoming route needs checking. Your arrangement may require an approved SIP routing configuration through a compatible PBX or SIP gateway.

Step 5: Import the number

1. Review the telephone number.

2. Check your SIP username and Service Password.

3. Confirm UDP is selected.

4. Confirm SIP registration is enabled.

5. Select Import.

Synthflow saves the custom telephone number and its SIP configuration.

Step 6: Assign the number to your voice agent

1. Open Agents.

2. Select the agent that should answer your Voyced calls.

3. Open the agent's Deploy section.

4. Find its telephone number assignment.

5. Select the Voyced number you imported.

6. Save or deploy the agent.

A single telephone number can be assigned to one inbound agent at a time.

Your voice agent now has a telephone number associated with it.

Step 7: Check the SIP registration

Return to Phone Numbers and open your Voyced number.

Check the available SIP connection and registration information.

Confirm that Synthflow has successfully authenticated with Voyced.

If the account does not register, check:

• The SIP username.

• The Service Password.

• The SIP Domain.

• The UDP transport.

• Whether SIP registration is enabled.

Do not proceed with live incoming calls until registration and call routing have been verified.

Step 8: Configure audio settings

Synthflow supports the following audio codecs for SIP trunking:

• G.711 A-law (PCMA).

• G.711 μ-law (PCMU).

• Opus.

For Voyced, start with:

1. PCMA.

2. PCMU.

Synthflow also supports DTMF through RTP telephone events and SIP INFO.

Use the default compatible DTMF configuration unless you encounter problems with keypad input.

Step 9: Check outgoing telephone number formatting

Synthflow stores imported numbers in E.164 format, normally beginning with +.

Voyced uses a different format for international calls.

All international calls sent through Voyced must begin with:

00 + Country Code + Telephone Number

For example:

These formats must not be confused.

If you use an authorised outgoing-call function or transfer that places a new call through Voyced, check the number actually sent in the SIP request.

The destination must follow Voyced's 00 format.

Synthflow's standard custom number settings do not document a built-in automatic conversion from + to 00.

If Synthflow sends the number with a leading plus sign, ask Synthflow Support about number rewriting for your SIP trunk before using that function.

Do not assume a successful SIP registration guarantees outgoing calls will use the correct number format.

Step 10: Check outbound Caller ID

By default, Voyced uses the authorised DID associated with your SIP account.

If you want to display another number:

1. Log in to your Voyced client area.

2. Open a sales support ticket.

3. Use the subject Outbound Caller ID adjustment.

4. Provide your Voyced DID.

5. Provide the number you want displayed.

Numbers outside your Voyced account require proof of ownership and additional verification.

Voyced Internal DIDs must not be used as public outbound Caller ID numbers.

Test Your Connection

Test 1: SIP registration

1. Open your imported Voyced number in Synthflow.

2. Check the SIP registration information.

3. Confirm Synthflow has authenticated successfully.

If registration fails, check the credentials before testing incoming calls.

Test 2: Incoming calls

1. Call your Voyced telephone number from another telephone.

2. Check whether Synthflow receives the call.

3. Confirm your assigned voice agent answers.

4. Speak to the agent.

5. Check that audio works in both directions.

If Voyced accepts the registration but Synthflow does not receive the call, review the incoming SIP route.

The registered destination must be able to receive the calls delivered for your DID.

Test 3: Agent behaviour

During an incoming test call:

1. Confirm the agent plays the correct greeting.

2. Ask a simple question.

3. Check that the agent understands the caller.

4. Confirm the response is audible.

5. End the conversation normally.

Review the call details in Synthflow after the test.

Test 4: Telephone keypad tones

If your agent uses keypad input, make a call and test the relevant numbers.

Confirm DTMF events reach the intended function.

Test 5: Outgoing calls and transfers

If your configuration includes an authorised outgoing call or live transfer:

1. Test with a permitted destination.

2. Confirm the destination number reaches Voyced in the correct format.

3. Check audio in both directions.

4. Verify the outbound Caller ID.

For international calls, the number sent to Voyced must begin with 00.

Do not use automated calling campaigns or predictive, power or bulk dialling features with Voyced.

Troubleshooting

SIP registration fails

Check:

Confirm your Synthflow plan includes SIP trunking.

Authentication errors

SIP response 401 Unauthorized may be a normal authentication challenge.

Repeated authentication failures may mean the username or password is incorrect.

A 403 Forbidden response indicates that the request has been rejected.

Check your credentials and the registration configuration.

Do not use your Voyced website login password.

Registration succeeds but incoming calls fail

Check:

• Your Voyced number is imported correctly.

• The number is assigned to the correct inbound agent.

• The agent is deployed.

• The incoming call reaches Synthflow.

• The Origination URI and SIP routing match your connection arrangement.

Registration and incoming routing are related, but they are not the same test.

If the call does not reach Synthflow, check the SIP routing between Voyced and the registered destination.

If the call reaches Synthflow but the agent does not answer, inspect its number assignment and call logs.

The agent does not answer

Open Agents and check that your agent:

• Is configured.

• Has the correct telephone number assigned.

• Is deployed.

• Has the required incoming-call configuration.

Review the failed call in Synthflow Logs.

No audio or one-way audio

Check:

• The supported audio codecs.

• The SIP connection and transport.

• The signalling and media addresses required by your Synthflow region.

• Any firewall rules affecting the SIP connection.

Start with PCMA and PCMU.

International outgoing calls fail

Check the number sent to Voyced.

It must begin with 00, not +.

For example:

00442035000202

If Synthflow sends an E.164 number beginning with +, the outbound number needs correcting before the call reaches Voyced.

Incorrect Caller ID

Check which Voyced DID is associated with the custom number.

Only use an outbound Caller ID authorised for your Voyced service.

Changing the displayed number in Synthflow does not automatically authorise it on Voyced.

Calls reach the wrong agent

Open the imported number and check its assignment.

A telephone number can be linked to one inbound agent at a time.

Make sure the correct agent is deployed.

Check call logs

Open Synthflow Logs and find the affected call.

Record:

• Date and time.

• Calling number.

• Called number.

• Incoming or outgoing direction.

• SIP error or failure reason.

• Whether the call reached the agent.

Provide these details to the relevant support team if needed.

Do not share your Voyced Service Password.

Final Tips

• Use sip.voyced.eu as your SIP server.

• Use UDP port 5060 for the standard Voyced connection.

• Enable outbound SIP registration when importing your custom number.

• Use your Voyced DID as the SIP username and authentication username.

• Start with PCMA and PCMU codecs.

• Import your number in the E.164 format required by Synthflow.

• Assign the imported number to your inbound voice agent.

• Test both SIP registration and actual incoming call delivery.

• Make sure any outbound international number sent to Voyced begins with 00.

• Use only authorised Caller ID numbers.

• Keep your use within Voyced's conversational voice traffic requirements.

• Do not use predictive, power, automated bulk dialling or short-burst calling traffic.

Once registration, incoming call routing and audio have been tested successfully, your Synthflow agent can handle calls through the verified Voyced connection.

Last updated: 28/10/2026

© Voyced
All information provided is copyrighted and purely informative in nature. It is maintained by Voyced with the greatest of care using reliable sources only.
Given the fast evolution of the telecom sector, Voyced does not offer any guarantee about the accuracy and completeness of the information offered. 
We explicitly do not give anyone permission to use this information in any other way than to inform themselves about what (legal) requirements there may be and they need to comply with when registering Services and/or DIDs with Voyced.