intercom using home telephone circuit

[interuser]

Hi ,
I have a 2 floor home and I would like and make an intercom using the
existing telephone circuit. Since every room has a telephone the
problem is making the telephones ringing. How would I do that?

 

[Dave]

In my house I used the 2 spare wires in the phone cable and added a
cheap buzzer by the phones. Connected at the calling phone was a
battery with a button that when activated sounded the buzzers at all of
the phones. The battery/button can be placed near as many phones as
desired as long as the polarity is maintained.
Dave