I have always assumed that when I make a voice call from the FaceTime app on my iMac, the call is being proxied through my iPhone. (In fact, the FaceTime UX says, Call Using iPhone; see attached image.)
However, today I tried an experiment: I switched off both my iPhone and my iPad--which has a cellular plan; I also turned off the WiFi on my iMac so that it was connected to the Internet only through an Ethernet cable to my router. I then used FaceTime on my iMac to call my landline. The call went through, reaching the answering machine hooked up to that phone.
My question is: How did this work? Does my iMac have some sort of VOIP system that it can use if it can't access a cellular device?
I am running macOS Big Sur 11.6.5 (20G527)