How can I build a device for making a stand-alone phone ring and operate for testing purposes?

I work in a small-town electronics store. Quite regularly, we have people bring in telephones or answering machines that are not working properly. Unfortunately, the telephone system in the store is business digital, which means we cannot directly connect the customers devices to it for testing purposes.

So, what I am wanting to do is construct a standalone device into which I can connect the phone and/or answering machine that will allow me to make the phone ring, and also allow the phone to operate when it is taken off the hook ("Operate" as in hear sound on the line, not necessarily call someone).

Now, I have already figured out how to make the phone ring, that was the easy part (A pair of transformers to step household 120VAC down to 10VAC, then back up to ~90VAC). The must-do tasks that still remain are:

-How do I make it stop ringing when I pick up the phone?
-How do I put an audio signal on the line that makes the phone work?

I'm new at this, so gentle/simple answers please.

The phone ring is usually 20 Hz, but 60 Hz will work fine as you note. When a phone is taken off-hook, the switching equipment at the telco premises sees this and removes the ringing voltage an applies 48 VDC for the voice part. The phone impedance off-hook should be about 500 ohms. On hook is about 1200 ohms if I am remembering right. If the phone rings, you no longer need to keep the 90 VAC applies. Remove it and connect the 48 VDC. You need to watch your polarity. One way will work and the other way won't. If the impedance changes correctly, and the audio works, the phone should be OK.

If you connect two phones and the 90v trfr in series with each other and you pick up both receivers, you will have something like an intercom system.

With cost of electronics so low these days, and cost of labor so high, I'm surprised that people would be willing to pay for repair rather than replacement. Still, check out this site for an off-line telephone tester, No phone line necessary.

http://www.uashem.com/pageid-305.html

Since you've already mentioned 120v ac, make sure you use a 120v to 24v CT (12v-0v-12v) transformer rather than the 230v one shown in the schematic.