You need to be subscribed to a voip service provider. Than you need to enter the following information into the softphone:
-Provider domain name or IP address
-Your username (this can be a normal phone number)
(you will get all this informations from the service provider)
That’s all, you can begin to call.
For example if you use <a href=”http://www.mizu-softphone.com/Download.aspx”> Mizu Softphone </a>, than you simply go to File Menu -> Basic Settings, enter this informations, hit the save button and start calling.