Users may encounter scenarios where specific SIP handsets or devices fail to register or place calls, while other devices on the same network/SSR branch function correctly. This article demonstrates how to use packet captures to determine if the issue resides in the SSR forwarding plane or the upstream SIP Provider.

Specific endpoints (e.g., X.X.X.X) fail to register.

SSR logs show traffic matches the expected services, but the application fails.

To isolate the root cause, a packet capture (pcap) was analyzed for two distinct devices.

The capture shows a healthy exchange of SIP OPTIONS between the provider and the local device (X.X.X.X).

• Transaction: OPTIONS -> 200 OK
• Conclusion: The SSR is correctly translating and forwarding bidirectional SIP traffic. The signaling path is fully functional.

The capture shows the device successfully resolving DNS and attempting to register, but receiving an error from the provider.

1. DNS Resolution: Device performs a NAPTR query for the registrar and receives a valid response.
2. Registration Attempt: Device sends a REGISTER request to the provider.
3. Error Response: The provider returns SIP/2.0 404 Not Found.

Note: Because the SSR successfully delivered the REGISTER request to the provider and returned the 404 Not Found response to the device, the SSR forwarding plane is healthy. A 404 Not Found is an application-layer rejection and not a data forwarding drop from SSR.