Treat me gently. it's my first post here

OK, I have a main site, this has a 2762 router with open ports to the BT (Avaya) IP office phone system, The two IP phones are pre-programmed to find the local IP if on the LAN and the IP of the router if they are outside.

I have a 2860n at a remote location with no ports opened, The IP phone on that LAN cannot contact the main system, If I use the same phone at home on a Virgin Media router it works perfectly, functioning as a remote extension with full features.

Question is, why is the 2860n not letting me connect? The fact that it works at home seems to back up BT who say that no configuration should be needed on the remote system.

Any advice much appreciated, this is driving me mad