The phone company applies about 90Vac @ 20Hz to ring the phone during
ringing, but they also have special circuitry to remove the ringing voltage
quickly when someone picks up the phone ("goes offhook").
It's possible that a phone might ring if standard 120Vac @ 60Hz (i.e.,
household power) is applied to the phone (the green and red wires), but this
would be VERY dangerous. If there were no circuit to either limit the
current, or disconnect the high voltage if the phone were to go offhook,
then the person using the phone could be harmed.
The easiest way would be for you to get a real phone line, for the play, and
then just call it (and hope that no one else calls it at the wrong time).
There are also telephone line simulators that you may be able to rent. How
about a tape recording of a phone ringing? A cellphone? Teach someone to
make a sound like a phone ringing?
Be careful! You want to make sure all the actors are still alive in case
there's a curtain call.
You might be able to put one of the cheap FRS radios behind or
close to the phone on stage, and then while holding down the
transmitt button on another, play a tape of a phone ringing into
it and the ring would come out of the one on the stage. Would
make an interesting speaker phone so the audiance could hear both
sides of the conversation. Wire up the cast with their own radios
with ear buds so they can be prompted when the lines are
If you're going to use an audio tape of a phone ringing,
just patch it into the PA, like they do in real studios. ;-)
Be sure the sound guy can see the stage, and cue the "click"
(and kill the bell) when the actor picks up.