Analog phone lines can easily induce signal on each other in areas where the lines are poorly insulated. If there's any analog switching equipment anywhere between handsets, one conversation can easily jump lines.
Ever seen one of these?
You plug the tracer into the wall, turn on the tone, and then wave the probe over this giant wall of shit
until the thing in your hand starts going "tweedleleedleeedleedleedle" and then you know which two of eighty gajillion wires go to that wall. But you never actually interrupt the circuit anywhere - that tweedling you hear is a magnetic field being generated over twisted pair.