Why does this setting prevent SIP trunk registration after Zendesk CTI migration?

Voice & Telephony
1

Why does this setting prevent SIP trunk registration after Zendesk CTI migration? We are in the final stages of moving our support operations from Zendesk Talk to Genesys Cloud. The ticketing integration is smooth, but the telephony layer is proving stubborn. In Zendesk, our CTI provider handled the SIP trunking with minimal configuration. Here, we are hitting a wall with the inbound voice flow.

We have configured the SIP trunk in the Admin panel. The IP address is whitelisted. The port is open. Yet, the status remains “Not Registered.” The error log in the Monitoring dashboard shows a 403 Forbidden response from the Genesys Cloud edge node when the carrier attempts to register. This is baffling because the credentials match exactly what was tested in the sandbox environment.

Environment details:

  • Genesys Cloud Region: eu-west (Ireland)
  • Carrier: Local French VoIP provider using standard SIP 2.0
  • Architect Flow: Simple inbound routing to a queue
  • Zendesk Legacy Setup: Custom CTI integration via third-party adapter
  • Error Code: 403 Forbidden on SIP REGISTER request
  • Timestamp: Occurs consistently every 60 seconds during retry cycle

In Zendesk, we never dealt with explicit SIP registration challenges. The third-party adapter abstracted this away. Now, we are exposed to the raw protocol details. We have verified the username and password multiple times. We have checked the network connectivity using telnet from our carrier’s test box. The connection is established, but the handshake fails.

Is there a specific security setting in Genesys Cloud that might be rejecting the REGISTER request? We have tried disabling the “Require TLS” option, but the error persists. We also checked the “Allowed IPs” list, and the carrier’s IP is definitely present. Could there be a mismatch in the SIP domain configuration? We are using the default Genesys Cloud domain for the trunk, but our carrier sends requests with a different Host header. Should we be mapping this differently?

Any insight into why the 403 is occurring would be incredibly helpful. We need to go live next week, and this blocker is causing significant stress. We are happy to provide pcap files if needed, but we prefer to solve this via configuration first. Thanks in advance for any pointers.

2

TL;DR: Check your SIP credentials and outbound proxy settings; the issue is likely a mismatch in authentication headers or a firewall blocking the specific SIP port post-migration.

The problem here is that the SIP trunk registration process is failing due to a misconfiguration in the authentication headers or a network-level blockage that was not present in the Zendesk environment. When migrating from Zendesk CTI, it is common to overlook the specific SIP credential requirements for Genesys Cloud BYOC trunks. The Zendesk environment may have handled SIP authentication differently, often abstracting it away, whereas Genesys Cloud requires explicit username and password configuration in the SIP trunk settings.

First, verify that the SIP credentials configured in the Genesys Cloud Admin panel match exactly with what your carrier expects. A common mistake is using the IP address as the username when the carrier requires a specific alphanumeric string. Additionally, check the outbound proxy settings. If your carrier requires a specific outbound proxy, ensure it is configured in the trunk settings. If the IP address is whitelisted but registration still fails, it is likely that the SIP REGISTER requests are being dropped by a firewall or NAT device that is not properly configured for SIP ALG (Application Layer Gateway). Disable SIP ALG on any intermediate network devices, as it can corrupt SIP headers and cause registration failures.

Here is a sample configuration to verify:

sip_trunk_config:
 username: "carrier_provided_username"
 password: "secure_password"
 outbound_proxy: "proxy.carrier.com"
 registration_interval: 30 # seconds

Ensure the registration_interval is set to a value that your carrier supports. If the issue persists, capture a SIP trace from both the Genesys Cloud side and your carrier side to identify where the REGISTER request is being rejected or dropped. This will provide insight into whether the issue is authentication-related or network-related.