Are you just giving away only IP addresses and the default gateway on your dhcp server residing on the Firebox?
Avaya phones need also DHCP option 242 to be set with a string that looks like this: "MCIPADD=10.100.0.10,MCPORT=1719,HTTPSRVR=10.100.0.10"
Where 10.100.0.10 is the IP address of the Avaya PBX.
In addition, if you don't have tunnels to all other IP Phone networks, you may run into a routing problem - a calling phone will want to contact the called one directly by it's IP address. If there is no tunnel/route to that address, the connection will go to the default gateway and end there, because there is no route to the other network.
My customers set the PBX so, that all calls are routed trough the PBX and not directly between each phone. Since the PBX has routes to all the IP Phones, this works perfect. Everything else may become a...
