As norstarboy125 mentioned - are you sure ALL of the appropriate ports are open in your routers?
Another possibility: Have you got any Cisco Routers or a Cisco PIX firewall in the network? There are some "gotchas" with Cisco/Nortel interop.
If Cisco Routers are in the network:
Cisco defaults with H245 tunneling enabled while BCM defaults with H245 Tunneling disabled. If you have Cisco Routers, either disable H245 tunneling on the routers or enable H245 tunneling on the BCM's. Enabled or disabled does not matter, they just have to match. If making hte change in the BCM's, stop/start VoIP Gateway service (or FEPS service) - whichever is applicable for your version of BCM.
If Cisco PIX Firewalls are in the network:
For H323: Disable the H323 fix-up protocols on the PIX (I believe there are two of them over ports 1718, 1719, 1720). If there is a PIX FW on each end, ensure this is done on both ends.
For SIP: Disable SIP fix-up protocol (port 5060) on the PIX.