Waiting a call hold can be boring if there is only silence on the line. It’s a good thing to present an “easy music”  in call hold, make a long waiting time feels much shorter. This simple circuit (consists of diodes, SCR, and some passive components) allows to place the caller on hold in one room then answer the phone again at another place, or at same phone at later time after doing something. This circuit is called music hold on circuit. This circuit will be turned off automatically when the phone is picked up the second time  and the conversation can be continued. Here is the schematic diagram of the circuit:

Music On Hold circuit schematic