LinuxQuestions.org - can i use VoipCheap with Twinkle and do phone to phone calls?

I ve just switched to Linux after many years of frustration with Windows...But know I need some help to get the things going that are important for my work. I used to use the Voip Cheap Software on windows and do phone to phone calls. My phone would ring and i pick up and than the other number would ring and they pick up. I have searched all day, read about softphones and sip calls and IP configurations etc. I'm still not sure if i understand right what is actually happening when I make a call with Voip Cheap that goes through my phone and I do not pay my phone provider for this. Hope this all makes sense to someone. Be great to get some help. BTW besides these little hick ups I am very much looking forward to be a Linux User. Thanks Mel_Portugal
My last try now was twinkle softphone and i thought maybe I have to redirect the call, but I don't know....