VoLTE/IMS: INVITE not routed when Request-URI port ≠ Destination-URI port (alias parameter ignored / destination mismatch in Kamailio PCSCF)

[nicthenothing]

When the Request-URI port and the Destination-URI port differ for a specific UE, the INVITE message fails to route correctly, and the call does not get established.
This occurs in scenarios where the “ear” channel and “mouth” channel (i.e., signaling ports seen from UE → PCSCF and PCSCF → UE) are not the same.

In such cases, Kamailio PCSCF appears to select the Destination URI from one socket while the Request-URI (which contains the correct alias and UE port) points to another. As a result, the INVITE is dropped or routed incorrectly.

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Key Logs

2025-10-15T13:21:33.638305291Z Destination URI: sip:10.20.7.216:42602;transport=tcp
2025-10-15T13:21:33.638307845Z Request URI: sip:3c7a3414-8cf7-458e-a674-9181ea0ccd15@10.20.7.216:42078

Example of another similar request:

Request-Line: INVITE sip:578da559-1f6e-430f-a953-8e4dfc99918e@10.20.7.228:40404;alias=10.20.7.228434262 SIP/2.0
Request-URI Host: 10.20.7.228
Request-URI Port: 40404

Here, the Destination-URI (42602) and Request-URI (42078) have different ports — leading to the INVITE never reaching the correct UE.

[herlesupreeth]

Please always attach a pcap taken for the scenario where you are facing the issue. The logs are not at all helpful in debugging.