VAPI integration | SignalWire

VAPI assistants need phone numbers, and SignalWire supplies them over SIP trunking in both directions. Callers dial your SignalWire number and reach the assistant, and the assistant places calls that show your SignalWire number as caller ID.

Each direction is its own SIP trunk in VAPI and its own SWML script in SignalWire. Set up the one you need, or both. The VAPI side follows VAPI’s own SIP trunk setup guide with SignalWire-specific values.

What you’ll need

A SignalWire account with at least one phone number (sign up here)
A VAPI account with API access
Your VAPI private API key (found in your VAPI dashboard)
For outbound calls, a SIP domain app password from SignalWire Support (requested below)

Inbound calls to your assistant

VAPI has to accept calls from SignalWire’s network, and SignalWire has to know which assistant a number belongs to. You create a trunk in VAPI that trusts SignalWire’s IP addresses, register your number with it, and then route the number to VAPI with a SWML script.

Create the inbound SIP trunk in VAPI

Run the API call below to create the trunk, making sure to replace YOUR_VAPI_PRIVATE_KEY with your actual API key.

Save the id from the response - you’ll need this credential ID for the next step.

Monitor SignalWire IP ranges

These IP addresses are current as of this guide’s publication, but SignalWire IPs can change and should be programmatically monitored to avoid any impact on calls. You can gather SignalWire’s latest IPs by performing a DIG or nslookup of sip.signalwire.com.

See our guide on allowing SignalWire IPs through firewalls for more details.

curl -X POST "https://api.vapi.ai/credential" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_VAPI_PRIVATE_KEY" \
  -d '{
    "provider": "byo-sip-trunk",
    "name": "SignalWire Inbound Trunk",
    "gateways": [
      { "ip": "170.64.128.96", "inboundEnabled": true },
      { "ip": "198.13.56.186", "inboundEnabled": true },
      { "ip": "104.248.176.184", "inboundEnabled": true },
      { "ip": "152.42.144.114", "inboundEnabled": true },
      { "ip": "104.248.150.114", "inboundEnabled": true },
      { "ip": "138.68.125.160", "inboundEnabled": true },
      { "ip": "159.65.244.171", "inboundEnabled": true },
      { "ip": "167.99.198.84", "inboundEnabled": true },
      { "ip": "13.245.35.235", "inboundEnabled": true },
      { "ip": "108.61.169.31", "inboundEnabled": true },
      { "ip": "137.184.4.155", "inboundEnabled": true },
      { "ip": "188.166.126.7", "inboundEnabled": true },
      { "ip": "139.59.34.94", "inboundEnabled": true },
      { "ip": "165.232.186.228", "inboundEnabled": true },
      { "ip": "157.175.131.128", "inboundEnabled": true }
    ]
  }'

Register your SignalWire phone number

Now register your SignalWire phone number with VAPI using the credential ID from above:

curl -X POST "https://api.vapi.ai/phone-number" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_VAPI_PRIVATE_KEY" \
  -d '{
    "provider": "byo-phone-number",
    "name": "SignalWire Inbound Number",
    "number": "+15551234567",
    "numberE164CheckEnabled": true,
    "credentialId": "YOUR_CREDENTIAL_ID"
  }'

Make sure to replace:

YOUR_VAPI_PRIVATE_KEY with your actual API key
+15551234567 with your SignalWire phone number in E.164 format (including the + and country code)
YOUR_CREDENTIAL_ID with the credential ID from the previous step

Assign your assistant

In your VAPI dashboard, find the phone number you just registered and assign it to one of your AI assistants. Configure the inbound settings only. If you also set up outbound calls below, you register the same number a second time against the outbound trunk.

Create the routing script in SignalWire

In your SignalWire dashboard, go to Resources → Add → Script → SWML Script. This script will handle all incoming calls by immediately connecting them to VAPI:

version: 1.0.0
sections:
  main:
    - connect:
        to: 'sip:%{call.to}@YOUR_CREDENTIAL_ID.sip.vapi.ai'

Replace YOUR_CREDENTIAL_ID with the credential ID from the VAPI trunk setup above. The %{call.to} variable ensures VAPI receives the original dialed number, which helps with call routing and analytics.

This SWML script uses the connect method to bridge the incoming call directly to VAPI’s SIP endpoint. The call happens in real time with no delays. Learn more about SWML in our complete guide.

Connect the script to your phone number

Go to Phone Numbers in your SignalWire dashboard, find your number, and click Edit Settings. Under the voice settings, assign your new SWML script to handle incoming calls.

Test inbound calls

Call your SignalWire phone number. Your VAPI assistant answers within a few seconds. If it doesn’t, the troubleshooting section below covers the common causes.

Outbound calls from your assistant

For outbound calls the roles reverse. SignalWire publishes a SIP address that VAPI dials, and a SWML script bridges each call onto the public telephone network with your SignalWire number as caller ID. VAPI authenticates to that SIP address with a password issued by SignalWire Support.

Create the outbound routing script

In your SignalWire dashboard, go to Resources → Add → Script → SWML Script. This script will handle outbound calls by connecting them through the PSTN:

version: 1.0.0
sections:
  main:
    - connect:
        answer_on_bridge: true
        from: +1A-Number-From-Your-Space-here
        to: '%{call.to.replace(/^sip:/i, '''').replace(/@.*/, '''')}'

Replace "+1A-Number-From-Your-Space-here" with an actual phone number from your SignalWire account. This will be the caller ID shown to people who receive calls from your VAPI assistant.

This SWML script uses the connect method with answer_on_bridge: true to ensure calls connect properly. The to field uses a regular expression to extract the phone number from VAPI’s SIP format and route it through the PSTN.

Add a SIP address to your script

After saving your SWML script, you’ll need to create a SIP address that VAPI can connect to:

In your saved SWML script, navigate to the Addresses & Phone Numbers section
Click Add and select SIP Address
Configure the SIP address settings and save

After configuration, note down your unique SIP domain app. It will look something like: test-space-vapi.dapp.signalwire.com

The Addresses documentation covers resource addresses in more detail.

Request the SIP domain app password from Support

To complete this setup, you must contact SignalWire Support to generate a password for your SIP domain app. VAPI needs this password for authentication.

Contact support by:

Clicking the “Help?” button in your SignalWire Space
Emailing support@signalwire.com

Provide your SIP domain app and let them know you need a password for VAPI integration.

Create the outbound SIP trunk in VAPI

Run this API call to create the outbound trunk, making sure to replace the placeholder values with your actual configuration:

curl -X POST "https://api.vapi.ai/credential" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_VAPI_PRIVATE_KEY" \
  -d '{
    "provider": "byo-sip-trunk",
    "name": "SignalWire Outbound Trunk",
    "gateways": [{
      "ip": "YOUR_SIGNALWIRE_SIP_DOMAIN",
      "inboundEnabled": false
    }],
    "outboundLeadingPlusEnabled": true,
    "outboundAuthenticationPlan": {
      "authUsername": "YOUR_SIGNALWIRE_PHONE_NUMBER",
      "authPassword": "YOUR_SIGNALWIRE_PASSWORD"
    }
  }'

Make sure to replace:

YOUR_VAPI_PRIVATE_KEY - Your VAPI API key
YOUR_SIGNALWIRE_SIP_DOMAIN - Your SIP domain app from the previous step (e.g., test-space-vapi.dapp.signalwire.com)
YOUR_SIGNALWIRE_PHONE_NUMBER - Your SignalWire phone number in E.164 format (e.g., +15551234567)
YOUR_SIGNALWIRE_PASSWORD - The password provided by SignalWire Support

Save the id from the response - you’ll need this credential ID for the next step.

Register your SignalWire phone number for outbound

curl -X POST "https://api.vapi.ai/phone-number" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_VAPI_PRIVATE_KEY" \
  -d '{
    "provider": "byo-phone-number",
    "name": "SignalWire Outbound Number",
    "number": "+15551234567",
    "numberE164CheckEnabled": true,
    "credentialId": "YOUR_CREDENTIAL_ID"
  }'

Replace:

YOUR_VAPI_PRIVATE_KEY - Your VAPI API key
+15551234567 - Your actual SignalWire phone number in E.164 format
YOUR_CREDENTIAL_ID - The credential ID from the trunk creation step

Assign your assistant for outbound calls

In your VAPI dashboard, find the phone number you just registered and assign it to one of your AI assistants. Configure the outbound settings to specify which assistant should handle outbound calls.

If you’ve already set up inbound calling with the same number, VAPI will indicate this is a duplicate number. This is completely normal and expected - you’re using the same phone number for both inbound and outbound calling with different configurations.

Test outbound calls

Your VAPI assistant can now make outbound calls through SignalWire. The calls will show your SignalWire phone number as the caller ID.

To test your setup, refer to VAPI’s official documentation on testing your SIP trunk for specific instructions on initiating outbound calls.

Troubleshoot the VAPI integration

You can monitor call logs in both platforms. SignalWire shows the call routing, and VAPI shows the assistant interaction details.

Inbound calls aren’t reaching VAPI

Check that your SWML script has the correct credential ID and is assigned to your phone number. Also, verify that your phone number is set to accept voice calls. Phone numbers must be in E.164 format (+1234567890) with no spaces or special characters.

VAPI can’t connect on inbound calls

Make sure all SignalWire IP addresses are in your VAPI inbound trunk. If SignalWire has added new IP addresses since you created the trunk, you’ll need to update it.

Outbound calls from VAPI fail to authenticate

Double-check that you’re using the correct SIP domain app password from SignalWire Support. The authentication username should be your phone number in E.164 format.

Outbound calls from VAPI don’t connect

Verify your outbound SWML script has the correct caller ID number and that it’s a valid number from your SignalWire account. Ensure your SIP domain app is properly configured in SignalWire and that VAPI can resolve the domain name.

Next steps

Outbound calling

Place outbound calls from SignalWire directly, including calls handled by a SignalWire AI agent

Advanced call flows

Add IVR menus, call screening, or business hours logic before connecting to VAPI

SWML connect reference

Every option on the method both routing scripts use