@puk : That's probably because of the way phone calls worked.Lines are now arranged in a trunk system, when you subscribe to a line you get a line and can call without operator assistance. Trunking also refers to the connection of switches and circuits within a telephone exchange.[2] Trunking is closely related to the concept of Grading. Trunking allows a group of inlet switches at the same time. Thus the service provider can provide a lesser number of circuits than might otherwise be required, allowing many users to "share" a smaller number of connections and achieve capacity savings.
Thus, the line could have been jumbled up , and you can start hearing the other call that runs parallel with yours.