We have retro rotary telephones, and I'd like to make a hold button for the one in my office.

I have fairly basic electronics skills but I don't actually know how the phone works, in that sense anyway.

Is this just a case of switching out the circuit to one playing "The girl from Ipenema" or is it more complicated than that?

For example, what is it that keeps the line open if I make an outgoing call? Call I accidentally cut the call or does the line need to go open circuit to do that? What are the voltage limits?