digital audio to phone line

/tech/, I come to you today with a challenge. I have done my fruitless research, and so, with nowhere else to turn, I ask you. On-Hold music units are suprisingly expensive. And are more than I need. What I need to do is convert output from a digital audio player into phone signal. Ideally: [audio player]-{standard 3.5mm double male cable}-[converter of some kind]--{phone cable}--[phone]--[jack]... I have looked for some kind of converter, and have found nothing. If such a thing is possible, I figured you guys would know how to make it, or where a guide to do so might exist.
Thanks for bothering to even consider the issue.

You'll need a phone patch to isolate the POTS voltage, http://www.rane.com/note150.html has a pretty good description. You may also need an amplifier, if so you can probably use a cheap headphone amp.

thanks! This looks like exactly what I need!