Twilio Elastic SIP Trunk¶

This guide covers setting up Twilio Elastic SIP Trunking as a PSTN gateway for the VoIP Server. Once configured, crew members can dial external phone numbers (mobile, landline) directly from the vehicle intercom system.

Prerequisites¶

A Twilio account with Elastic SIP Trunking enabled
A purchased or ported phone number (DID) for caller ID
The vehicle's public IP or VPN endpoint accessible from the internet
VoIP Server running with the Twilio gateway plugin (libgw-twilio.so)

Setup Steps¶

1. Create a SIP Trunk¶

Log in to Twilio Console
Navigate to Elastic SIP Trunking → Trunks → Create new Trunk
Set a friendly name (e.g. "GVA Vehicle Trunk")
Under Termination (outbound calls):
- Set the Termination SIP URI (e.g. your-trunk.pstn.twilio.com)
- Enable Credential List authentication
Under Origination (inbound calls):
- Add your vehicle's public IP or VPN endpoint as an Origination URI
- Format: sip:<your-ip>:5080;transport=udp

2. Create a Credential List¶

Go to Elastic SIP Trunking → Authentication → Credential Lists
Create a new credential list
Add a username and password — these go into voip-settings.json

3. Assign a Phone Number¶

Purchase or port a phone number (DID) under Phone Numbers
Configure the number to route to your SIP Trunk
Set the callerIdNumber in the gateway config to this number (E.164 format)

4. Configure the Gateway Plugin¶

Add the Twilio route to your voip-settings.json:

{
    "gateways": {
        "routes": [
            {
                "plugin": "libgw-twilio.so",
                "prefixes": ["+", "00"],
                "config": {
                    "trunkHost": "your-trunk.pstn.twilio.com",
                    "trunkPort": 5060,
                    "transport": "udp",
                    "username": "your-credential-list-username",
                    "password": "your-credential-list-password",
                    "callerIdNumber": "+441234567890",
                    "allowedIps": [
                        "54.172.60.0/23",
                        "54.244.51.0/24",
                        "34.203.250.0/23",
                        "54.171.127.192/26",
                        "35.156.191.128/25",
                        "54.65.63.192/26",
                        "54.169.127.128/26",
                        "54.252.254.64/26"
                    ],
                    "codecs": ["PCMU", "PCMA"]
                }
            }
        ]
    }
}

5. Configure Allowed IPs¶

Twilio's signalling IPs must be whitelisted so the server can validate inbound trunk traffic. The allowedIps list in the config above contains Twilio's current IP ranges. Check Twilio's documentation for the latest ranges for your region.

Testing¶

Start the Server¶

./build/bin/gva-voip-server --domain=0 -f /etc/gva-voip-server/voip-settings.json

Place an Outbound Call¶

Dial a PSTN number from the VoIP client. The + or 00 prefix routes through the Twilio gateway:

Dialed: +441234567890

Verify in Logs¶

Check the server logs for gateway activity:

[gateway] Twilio: Ready (trunk: your-trunk.pstn.twilio.com:5060/udp)
[gateway] Routing +441234567890 via twilio (prefix: "+")
[gateway] Twilio: 100 Trying
[gateway] Twilio: 180 Ringing
[gateway] Twilio: 200 OK (call connected)

Verify in Twilio Console¶

Navigate to Monitor → Logs → Programmable Voice in the Twilio Console to see call records and any error codes.

Troubleshooting¶

Symptom	Likely Cause	Fix
`403 Forbidden` on outbound	Credential mismatch	Verify username/password match Twilio credential list
No audio after connect	NAT/firewall blocking RTP	Ensure UDP 10000-10100 is open; check external IP
`408 Request Timeout` inbound	Trunk can't reach server	Verify Origination URI and firewall rules for port 5080
`488 Not Acceptable`	Codec mismatch	Ensure `PCMU` or `PCMA` is in both trunk and client config
Calls drop after 30s	Missing ACK/keepalive	Check NAT timeout settings; enable SIP session timers

Security Considerations¶

IP Whitelisting: Always validate inbound trunk traffic against Twilio's published IP ranges
TLS Transport: For production, consider transport: "tls" with Twilio's secure trunk endpoints
Credential Rotation: Rotate Twilio credentials periodically
Rate Limiting: Configure max concurrent trunk calls to prevent toll fraud

Twilio Setup