Connect an AI voice agent to Cloud SIP

How an AI voice agent connects to Cloud SIP

An AI voice agent platform, such as ElevenLabs, connects to Cloud SIP the same way a phone system does, in two directions. Set up the direction you need, or both.

E.164 is the full international format for a phone number, such as +61255501234.

Send incoming calls to your agent

Your phone number's incoming call handler forwards each call straight to the platform, so the caller's number reaches your agent unchanged.

1. In your agent platform, add your Enfonica phone number in E.164 format, for example +61255501234, and assign it to your agent. The platform uses the number to decide which agent answers.
2. Copy the platform's SIP address for incoming calls. Platforms call it the SIP server, SIP URI, or SIP endpoint, for example sip.agents.example.
3. In the Enfonica Console, go to Phone Numbers > My numbers, and open the number.
4. Under Call settings, in Incoming call handlers, select Add, and then select SIP.
5. Select SIP URI, and enter the platform's address with TLS, for example sip:sip.agents.example;transport=tls. Leave out the part before the @. Cloud SIP fills in the number that was called, in E.164 format.
6. If the number has another handler from before, delete it. To keep your phone system as a fallback, use Flow instead. To learn how, see Answer with your agent first, then your team.
7. Select Save Changes.

Cloud SIP connects over TLS (port 5061) or UDP (port 5060). It doesn't support SIP over TCP, so choose TLS whenever the platform offers it. With TLS, Cloud SIP also offers encrypted call audio (SRTP). If the platform requires encrypted audio, add ;secure=true to the address.

Let your agent make outgoing calls

The platform sends outgoing calls to your SIP domain, which is your trunk on Enfonica. Every call carries a username and password that proves it's yours.

1. In the Console, go to Cloud SIP, and open your SIP domain. Or, to create one, select New SIP domain, and create a Standard SIP domain. A separate SIP domain for your agent keeps its settings apart from your phone system's.
2. Under Termination settings, set Call termination mode to PSTN Trunk, so that calls go to the phone network.
3. Under Users, select Add user.
4. Enter a username. For the password, enter at least 12 characters with uppercase letters, lowercase letters, and digits, or select Generate.
5. Keep a copy of the password, and then select Add.
6. Under IP access control list, delete any IP ranges.
7. Select Save Changes. Changes take effect within about a minute.

Next, enter these values in the platform's outbound SIP trunk settings.

Numbers your agent dials work best in E.164 format too. National format, such as 0255501234, is also accepted. To learn how to send 13, 1300, and international numbers, see Which number format should my PBX use?

Transfer calls from your agent to a person

Choose a transfer type that makes a new call and joins the two calls together. Cloud SIP doesn't support SIP REFER transfers, so a REFER from the platform is refused with 405 Method Not Allowed.

In ElevenLabs, set the transfer rule's Transfer Type to Conference, which is the default. The platform then places a second call to the person through your outgoing trunk, connects the caller, and leaves the call. Other platforms have a similar option, often called a conference, bridge, or warm transfer.

Transfers work only after you set up outgoing calls, as described in the previous section. Each transfer is an outgoing call, so it counts toward your plan's outgoing channels. For more information, see Some calls fail when you're busy.

Answer with your agent first, then your team

To have your agent answer first, with your team as the fallback, route the call in Enfonica, not in your phone system. When a phone system forwards a call out to an agent, many show your number instead of the caller's. Routing in Flow keeps the original caller ID.

1. In the Console, select Flow, and create a new flow.
2. Drag a Call step under When a call is received.
3. Under Recipients, select Add recipient, and then select Add SIP URI.
4. Enter the agent's SIP URI, for example sip:sip.agents.example;transport=tls.
5. Add a second recipient the same way, with your phone system's load balancer, for example sip:pbx@example.sip.enfonica.com. For more information about load balancers, see Send incoming calls to more than one PBX or site.
6. Under Advanced controls, make sure that Strategy is Linear, which tries the recipients in order.
7. To reach your team sooner when the agent doesn't answer, lower the Ring time.
8. Select Publish.
9. Go to Phone Numbers > My numbers, and open the phone number.
10. Replace the number's handler with Flow, select your flow, and then select Save Changes.

Your phone system now receives these calls only when the agent doesn't answer. Make sure that it doesn't forward them back to the agent.

Check that it worked

1. Call your number from a mobile. Your agent answers.
2. In the Console, go to Cloud Voice > Calls, and open the call. Its SIP Insights show each message between Cloud SIP and the platform. To learn how to read them, see Find a call and read its SIP trace.
3. Have your agent call your mobile. Your mobile rings and shows the agent's number.
4. Ask the agent to transfer you. The person you transfer to answers, and the agent leaves the call.

If an outgoing call fails, its response code points to the cause:

• 403. The username, password, or IP list.
• 405. A REFER transfer or a registration attempt.
• 484. The caller ID or the number dialed.

For each code and its fix, see Outbound calls aren't going through.

Questions