Unexpected IP / Source Port change
The remote device behind the NAT (terminal / gateway) within the same session has changed the source IP / Port.
If the duration of the UDP NAT sessions is configured correctly, i.e. greater than the registration period, each device will send a request before the NAT session expires which therefore will never change. Each device will be seen to arrive permanently from its own public IP pair: UDP port. If, on the other hand, there is a frequent change of the source port, it is very likely that the router is assigning a different UDP port each time because the NAT session used previously has already expired.
The converse may not apply, that is, a router with too short NAT sessions could still reassign the same ports used previously, especially if each device uses a different source SIP port. So a frequent port change is certainly an indication of a badly configured NAT, but the latter may not necessarily give rise to a port change.
- Duration of NAT sessions too short compared to the registration period of terminals / gateways
- Check that the duration of the NAT sessions for the UDP protocol is at least 60 or 120 seconds, depending on what is set on the terminals / gateways