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
