IPS Service seems to block incoming SIP calls - SFOS 18.0.0 EAP3

What feature is impacted?
IPS Service seems to block incoming VoIP calls

What is the severity of the issue?
Medium

Summary of the issues:
IPS Service seems to be blocking incoming SIP calls.

Observed behavior:
After an internal PBX (AVM Fritz!Box) behind the XG has registered to a VoIP Provider (Deutsche Telekom), Outgoing calls are working as expected. But incoming calls are not signalled to the PBX. Stopping the IPS Service solves the problem.

Desired behavior:
Incoming VoIP Calls should be working even with IPS Service being started.

Other:
While doing the Tests, all filters have been disabled for the PBX traffic.

The corresponding Firewall and NAT Settings are as follows: