You do not need a hardware IP phone, though it can be used. You can download any free software Voip phone to make calls. You will have to provide the IP provided as the domain and also provide userId and Password in the configuration of the software IP phone. There are a lot of VoIP phone in Linux, one of them is Ekiga.
I use X-Lite to make calls.
I have a problem here, i want to make calls from other PCs connected to my machine.