After checking out the Tele-Q, take a look at Theatrical Reality's TRG-10K Programmable Telephone Ringer.

http://www.theatricalreality.com/

Nearly the same price (when you figure in the power pack), but a WHOLE lot more for the money.  It was designed specifically for the stage, so you can speed it up or slow it down.  It will ring with either a US cadence or that of the UK, AND once you start it (a single push of a button) it will automatically ring the phone continuously until either your actor picks up the receiver or the button is pushed again.

Sorry for the shameless promotion, but if you want to ring a real telephone on stage, I think this will do the trick better than almost anything out there.

I have the good fortune to have a husband who  works for Bell.  Every time one to the CT's in town needs a phone to he sets it up so the phone on the stage actually rings and stops when the actor picks up the receiver.  Just requires some kind of ringing generator that he plugs in and runs the phone cord to.  It's even more fun when we rig a phone at the other end and talk to the actor on the phone.   THE SM just needs to push a button at their end for each ring.