I can't speak specifically for your issue with FWD Communicator, except to say I find that client to deficient. On my setup the Communicator continually fails to 'attempt to use STUN' and then logs in using the outbound proxy service, which is not what I want.
I can recommend two free Windows SIP softphones that can be configured to run FWD, Gizmo Project, and Voxalot accounts:
1) X-lite 3.0:
http://www.xten.com/index.php?menu=Products&smenu=xlite
2) SJphone:
http://www.sjlabs.com/sjp.html
The SJphone permits multiple registrations to different providers, while X-lite is only to one, but X-lite is a lot more polished looking.