Hello, Zoiper itself enough for making calls, you would need to set a SIP or IAX account from a VoIP service provider, since Zoiper does not come bundled with service. It is only a softphone application.
The fees for the calls will vary according to the VoIP providers pricelists.
It is the same as buying a GSM Phone device. You would then need to find a service provider and you will be paying for your calls to the service provider.