Need some help troubleshooting SIP Trunk 503 Service Unavailable

Voice & Telephony
1

Need some help troubleshooting

Background

Migrating from Zendesk Talk. Configuring a new SIP trunk in Genesys Cloud (EU-West).

Issue

Registration succeeds, but outbound calls fail immediately with SIP 503 Service Unavailable. Inbound works fine. This feels like a routing mismatch, unlike Zendesk’s simple direct line setup.

Troubleshooting

  • Verified SIP URI matches exactly.
  • Checked outbound_route in Architect.
  • Firewall ports 5060/5061 open.

Is there a specific header mismatch common during migration?

2

This looks like a routing mismatch or a capacity issue on the trunk side, especially since registration works but outbound fails. From a load testing perspective, 503s often appear when the downstream provider rejects the INVITE due to missing or malformed headers, or if the trunk is hitting a concurrent call limit immediately.

Check the outbound routing rules to ensure the SIP trunk is selected for the correct destination patterns. Also, verify the trunk configuration payload. A common mistake is omitting the sipUri or misconfiguring the transportProtocol.

{
 "name": "Primary SIP Trunk",
 "sipUri": "sip.provider.com",
 "transportProtocol": "TCP",
 "outboundProxy": {
 "host": "proxy.provider.com",
 "port": 5060
 },
 "enabled": true
}

Ensure the outboundProxy matches your provider’s requirements. If this config is correct and you still see 503s, check the trunk’s concurrent call capacity settings. It might be rejecting calls if it thinks the limit is reached, even during initial setup.