This is quite an uncommon problem.
The problem lies, not in your PC, but in the actual exchange itself.
You will need to talk nicely to your telephone company and get them to install a new exchange in your room.
Either that or you can fork out for a t-line microfilter. This will hopefully fix the problem as it converts your line into two different lines, then buy a private exchange from a local market. Plug the modem into both of the exchanges (the one you already have and the one you have just bought, and this will serve to give you the one line that you need and the other dormant line will act as if nothing is plugged into it at all, thus stopping the ringing.