1) Check if firewall have any issue with VOIP. Make sure traffic for VOIP IP address enable fully.
2) MTU size must be 1400
3) Check NAT issue the Firewall
Note: Many times these segments (Q.931 SETUP) are not passing thru the routers end-to-end to the gatekeeper. If network gears are not properly set up they may either discard these segments or if they do pass them along not necessarily process them the correct way hence the Q.931 SETUP will not be recognized and registration will not succeed.
This is a typical network admin issue when packet segmentation occurs over remote/WAN (low bandwidth) connections.
