Well if I was going to do it.... Possibility #1: I would build two hi-z bridging circuits, one for each end. Follow that with two tone decoders or one DTMF decoder. Set decoders for or use # or * outputs. Press # or * on either end and audible goes off on both ends.
Possibility #2: Build a hi-z bridging DC voltage detector circuit for both ends. Also use a button with a loading resistance on each side. On hook voltage would be the highest. Check typical off-hook voltage with one and then both phones off-hook. Set the detectors to drive a piezo a volt or so below the voltage measured, but not at zero volts. (Unless you want both audibles to go off if you lose your phone lines. Pick the series resistance with the N.O. pushbutton to load the phone line below the threshold of the detectors.
But that's just what I would do if I had only one active line. But if you have 1 or 2 free pairs and a couple wall warts, there are many other opyions. Joe
Use the AC powered series circuit with the diode across each switch and each lamp. Replace the lamp with a DC coil relay to drive an audio alarm at each end. Power it with a wall wart and you are done.
Most office products stores sell AT&T 4-line phones with intercom capability. They can be used with one single line, as long as it is on line one. As long as the wiring between them isn't more than 600 feet, they will allow calls to be answered, placed on hold and picked up at stations in other locations or buildings. They also offer intercom which is actually FM riding along the cable pair being used for line 1.