The tech behind wired voice telecom is essentially over 100 years old. Everything since is just ways to aggregate the links.
Yea, if you have 2 handsets, you can wire two lantern batteries together in series and have a private line. Throw in a few dollars in transistors, transformers, and circuitry and you can even make it ring on the other end when you pick up the handset.
If ever society collapses a bit of work can turn a dead phone system into a neighborhood alert network.
You do not need any batteries. The microphone in the handset can generate enough to power it for sometimes up to 5 miles. The old “Sound Power” handsets had two alligator clips that were used to communicate for linemen to get the right wires connected.