Sam,
I have just checked the Webphone from Freeserve. But unfortunately there is
no info which ports are used for media traffic.
The signalling traffic uses port 5061 (TLS over TCP). But this port is not
the problem in your case.
The signalling connection on this port is obviously opened by your PC. But
the incoming media traffic seems to be blocked by ICF as you already
suggest.
Now the only viable ways to get the port number is to ask Freeserve for it,
and hope that the port number for media is static or a small range of port
numbers.
Another chance to get the port number is via tracing the traffic if you have
a tracing tool.
You can than see the media traffic leaving your PC. I think that the
incoming traffic will use the same port number as the outgoing media
traffic.
Good Luck.
Franz