I'm no expert with networking (hence why I'm posting a question on
here!), and my problem is as follows:

I have a Cisco 7960 phone with the latest SIP image on it. I have
configured it to connect to the Nildram VoIP service. I can make calls
fine, and have two-way sound (SOME of the time), but I am unable to
receive calls at all.

I'm pretty sure it's a configuration option on the phone, but not sure
what exactly. The reason I say this is because I've also configured
the X-Lite PC software to use the Nildram VoIP service and that can
receive calls fine, so I *think* this rules out a router configuration
problem possibly...

I have spoken with Nildram VoIP support who have told me they couldn't
offer support for my setup and couldn't suggest anything else to try
apart from adjusting the port forwarding rules on my router.

My setup is as follows:

2Mbps ADSL connection, with a 3Com Router (3CRWE754G72-A), and PC's
and the Cisco 7960 IP phone connected directly in to the Router.

I have tried the Router with the firewall disabled, adding the IP
address of the phone to the DMZ. During both of these changes I am
able to make calls fine (again, most of the time - sometimes the call
is completely silent), but still not able to receive any incoming
calls (if I call the number I get a "person is unavailable" message
and it goes to the answerphone service).

After finding a few things on the Internet I have tried adding the
following rules to the router:

Forward ports 5060 TCP and UDP to IP of phone
Forward ports 5082 UDP to IP of phone

Although I don't think forwarding these ports would make much
difference because I can make calls fine.

I only have some general knowledge of networking, but think that's
reached it's limit!! If anyone could suggest how I can receive calls,
I would be extremely grateful!

Thanks