Please don't think I'm trying to talk down to anyone, but in case some don't understand the concept of the gossip phone, let me explain.
Since Main Street is themed around the turn of the 1900's, phones at that time were wired on an open line that was shared with many other households (for those who know more about phone systems than I, please forgive any inaccurate terms). It was very common to pick up the receiver, and hear someone else talking. They could hear you, as well, if you made any noise.
Once again, Disney wanted one more small detail in place to present the proper theme in Main Street.