SIP capable program for VoIP provider

Hello

Does anyone know a linux-forum for the following problem:

I use the VoIP provider internetcalls.com. For windows they have a program which works for making calls.

Now for linux there is none of such. But one could use a SIP capable program to connect to sip.internetcalls.com with its own account.

I tired linphone and Ekiga but connection to internetcalls.com fails

Has anyone experience with such stuff or does somebody know a forum which handles such kinds of topics?

with regards