Media for The simplest and cheapest PBX-like device?

The simplest and cheapest PBX-like device?

I am trying to find a communications solution for two elder relatives of mine. They live next to each other in a place where phone calls are expensive, even local ones. I have considered a couple of "intercommunicators" which have to be plugged only to the AC power, and the voice is modulated by the 110v. signal, with no need for a data cable. The problem with this solution is that the two homes don't share the electrical circuit. I would like the communication to be full duplex, too. Are there full duplex wireless phones?

Finally, what about a tiny, inexpensive, 2 line PBX? Is there such a thing?

Thanks so much for your kind assistance, my two aunts will appreciate it, too.

-Ramon