You need 2 or 3 things to do that. On the system cabinet is a port labelled MOH. (There may be 2). You need a music source, in the U.S. you can use a commercial grade player with promotions. Use a search engine with the words MUSIC ON HOLD PLAYER. You could also use a radio, just don't get caught. Especially in today's climate, people make a living calling your business and listening for illegal use of commercial radio on phone systems. The licensing agencies issue huge fines for that.
Last, you also need some programming in system options.
Here's a reprint from another forum:
If you're looking for the best all-around player, the IntelliTouch OHP-6000 and OHP-8000 is a good place to start. It comes with Message Studio 3.0 that gives you the ability to mix royalty free music
with your voice cuts. You can also purchase about 30 more tracks for $20. You can download Message Studio 2.5 to test the interface and search the product at www.onholdsystems.com
The 6000 uses SD Memory, the 8000 uses USB memory drives.
I am also very fond of the Premier Technologies USB-1100. It's more expensive but it's all metal construction, easier to mount, and uses the USB drive. You transfer your MP3 files to it, plug it in, hit the reset button and walk away from it. They're at http://www.premtech.com/usb1100.html
You don't get software with this unit, but I happen to know someone who keeps a copy of Message Studio 3.0 on his laptop for these occasions and we can sell you Message Studio 3.0 alone.
Hope this helps.Carl