Have a look at the DSL modem and note it's make and model, e.g. "Alcatel". When you have this, do a search on the forum and see if anyone else is using this modem and see what they needed to do to get it working, there is a very good chance someone else has done what you are trying to do before.

If that doesn't help, try google, I'm sure you have a good chance of getting it to work.

Cheers, Stuart.....