There is an article in the  'How to' section of this website on this topic (http://www.communitytheater.org/howto/phone.htm). If using a 30Hz signal doesn't work, you might want to try 20Hz. You should be fine either way though. Note: this solution requires your light/sound/stage person to stop pushing the button once the person picks up.

As an alternative, I had done this a bit differently. I created a 20Hz tone on my computer (with the appropriate ring cycle spacing), and burned it onto a CD. If you do it this way, you can use your CD player to ring the telephone. You hook your telephone wire directly to the amplifier as if it were a speaker. When you do this, your CD player + amplifier behaves as a signal generator. All you have to do is press 'play,' and your phone starts ringing.

A few caveats: 1) You still have to have a tech press 'stop' when the actor picks up the phone. 2) Since the phone signal is a 90V, 20Hz signal, you will have to turn the volume up on your amplifier until you find the level needed to make the phone ring. 3) Your little portable CD player isn't strong enough to ring a telephone, you need an actual amplifier to make a 90V signal. 4) A pushbutton solution is easier to use, I had put this together because it didn't impact the budget for a show I was working on.

-Tom

P.S. Oh, and make sure that you don't actually have speakers on line when you actually hit 'play.' I did this once by mistake while doing a pre-show equipment check. Holy cow! If you actually have speakers good enough to hit 20Hz, it's a real trip. Several actors thought we were having an earthquake! (Do not try this at home, or in the theatre. It's very likely that you can ruin some very good speakers this way.)