I've been trying to figure that out myself. Another post here recommended Grandstream products for connecting analog phones to VoIP. Looking at the user guide for the HT802 you can make direct calls between the extensions. http://www.grandstream.com/products/gateways-and-atas/analog...
Unfortunately there system requires dialing "*701" or "*702" and dialing star on a true rotary phone isn't possible without some other system in place. Like this product from dialgizmo: https://www.dialgizmo.com/how_it_works.html Dialing two consecutive stars would still be annoying and slow.
I've been wanting to wire up an in-house "intercom" system using old phones. I'm a little surprised there isn't a "telephone HAT" for the raspberry pi that will interface directly with an old phone. A HAT that could interpret the DTMF or pulse dialing and drive even the old fashioned mechanical ringers.
I was thinking that there should not be any huge technical problem with making an FXS interface for the raspberry pi, and I did find a USB adapter which I might have been tempted to to try, but it's a little expensive and only supported 0.7 REN (ringer equivalency number) which might not be enough to power the mechanical ringer on an old phone. Further searching did turn up the OAKR2 HAT though ( https://switchpi.com/oakr2-module-specifications/ ) and it looks like the schematics are on github.
The idea is to pick up an old rotary phone and hook it up a raspberry pi which will make phone call over VoIP. The rotary dial will just be the user interface, the mic and speakers will be hooked to the RPI.
There are some project on github explaining the project: https://github.com/hnesland/aselektriskbureau
I plan to enhance it a bit to use the original ringer with the help of some power electronics.
However I have no clue on how to create a Voip server at home to serve phones locally and assign phone numbers.
Unfortunately there system requires dialing "*701" or "*702" and dialing star on a true rotary phone isn't possible without some other system in place. Like this product from dialgizmo: https://www.dialgizmo.com/how_it_works.html Dialing two consecutive stars would still be annoying and slow.
I've been wanting to wire up an in-house "intercom" system using old phones. I'm a little surprised there isn't a "telephone HAT" for the raspberry pi that will interface directly with an old phone. A HAT that could interpret the DTMF or pulse dialing and drive even the old fashioned mechanical ringers.