Okay, I was going to go with VoIP, but I bought a router with 2 phone ports! Sounds like this is software-based. I just need a microphone and speakers to sign up?
Btw, about 6 years ago I was teaching seniors in a private retirement community to use VoIP, on PCs in their special broadband area. I taught use of Net2phone (I think that was what it was called) and another, with a name I've forgotten. These were both software-based, using PC's speakers and mike. All calls were completely free, including PC-to-phone calls, but you had to register for the services.