I have a problem with firewall that I cannot replace or reconfigure. The FW is a SonicWall, public IP on Internet side (of course), NAT on the LAN side, gatekeeper on a public address outside FW. Calls from EPs on NAT to outside world work but incoming calls fail. I wiresharked the traffic and found that the Q.931 setup ServiceControlIndication packet bounces from the FW. The reason for this is that while this particular packet is sent to UDP port previously used by the NATed EP RRQ, the FW fails to preserve NAT mapping table. SonicWall  also ignores port preservation setting.