If you are behind a NAT

The phone uses UDP for communication. UDP is connectionless, and so doesn't get on well with NATs. If it is at all possible, plug in the phone to the Internet side of the firewall with its own real, proper, globally routable IP address (not a private IP address).

If you must work behind a NAT, this is where it gets tricky. You need to tell the phone that it is behind a NAT, tell it what its external IP address is. 1:1 NAT is better one method. Alternatively you can use the DMZ facility of some NATs or you can do some port mappings.

If the NAT claims to be H.323 aware, then you are probably best to disable this feature. It may help you for outgoing calls, but is probably going to cause pain for incoming calls. Some H.323 aware NATs only seem to recognise voice, and not video.

Port numbers

These are the port numbers you need to map through if you are using the phone behind a NAT.

TCP: 7000-7009, 1720-1730
UDP: 8050- 8054, 8056, 8058

The port numbers printed in the user manual are incorrect for newer software version.