You could take pretty much any speaker you wanted to, run a jumper to the switch and listen to any phone call you wanted.
Back in the day I had a friend who worked for a phone company. As a part of their QC they had a speaker in their office that was connected to various random landlines in order to keep a check on call quality. (In hindsight the choice of lines is a bit suspect)
He told me a story of one call they heard, about the mother of little johnny calling up a phone sex line (but not knowing it was one) and wanting to know what all these charges were for. The phone sex worker tried to sidestep the issue by saying that little johnny was just talking to "friends". The mother replied "but little johnny has lots of friends". At that point the phone sex worker drop all pretense as to what the calls were about, and the last thing the mother said was "wait until little johnny's father gets home"