Apologies if I'm not clear but this is pushing the limits on my meagre technical expertise in VOIP.

I presume if a friend uses a service which lets you specify a sip address for a call (e.g, the gizmo project), they will be able to use nnnNNNnnnn@sip.voipwelcome.com (where nnnNNNnnnn is my ten digit phone number starting with area code) to contact me on my VOIPo line. Assuming the above is correct, here's the question... if they do not use such a service is there any kind of client they can use (on their windows machine) which will let them make a call?