From what I can tell, you want to plug a phone into your voice modem, and have the voice modem make that phone ring, right?
Unfortunately, a voice modem is unable to do this. IIRC, a phone rings when you put 90VAC on it, but depending on the phone will still ring as a lower voltage. I don't know about the frequency, but it shouldn't matter, 20Hz that Frog suggested will do. A modem isn't able to generate the kind of voltages required to make the phone ring like this, and indeed, doesn't have the hardware support to do it.
This explanation gets more technical and closer to a true ring the further down you read. Just use the bits you need for your application.
My suggestion, for a cheap, ghetto-tech solution is building a chopper-relay based system, having a relay chop an electrical supply, and running the chopped supply through a step-up transformer to give the required ring voltage. Note, that most phones require AC to ring, so chopped DC won't do it.
A better, and more natural-sounding system would be to build a multivibrator circuit driving a couple of small power transistors in a push-pull setup. Center-tapped transformers are great for a simple design system, by having the transistors supply each side of the transformer with power in an alternating pattern, and grounding the center-tap.
Another good thing to do is to build a circuit to turn the ringer on and off to imitate the particular ring pattern of the area you are using this device in. Depending on the ring type, you may be able to get away with something as simple as an astable multivibrator (ring, long pause, ring, long pause...) or you may need something more involved, perhaps a design using "555" timers to make that "ring, ring, pause, ring, ring, pause" type of ring. You could just turn your machine on and off to simulate the ringing pattern, but sometimes (unless you can get it right) people may be able to tell the ring is slightly different than what it normally is.
Don't forget that when your target picks up the phone, the line resistance goes low, because the audio transformer in the phone is cut into the circuit (signaling the end-office that the subscriber has picked up the phone). If the ring were to continue after the person picks up the phone, you can bet they'll notice it. It'll be fairly loud in the ear-piece, and will sound like a loud sharp ticking noise or a strong buzz.
You'll need to get the ring frequency close if you want it to sound exactly like the phone company's ring, too. (Example, ringing-bell style phones with the old bell and striker ringing will sound different unless the frequency matches.)
If you absolutely MUST have it computer controlled, look into getting some hardware to use with a soft-PABX style system. I know there are Asterisk systems that use such hardware, but unless you run an Asterisk or other soft-PABX system on the computer, you will need to write a program to access the hardware and make the phone ring.
EDIT: Added stuff about ring types and frequency.
