Caius, from what we talked about on IRC.

1. Double and triple check to verify that someone isn't picking up the phone line, and that you've disabled call waiting when you dial the ISP. It's *67 in the US, I don't know what it would be for your country's phone system.

2. Pick up all of your phones and listen for line noises. Lots of popping and crackling, static etc. If you hear a lot, disconnect all of your phones, then plug one in that you know is good. If you don't hear any noise, it could be something like one of your other phones (or even your PC) that is causing the noise. Try to isolate this. Your modem inside your PC might even be going bad. Also, if you're brave enough, there should be a demarc on the side of your house where the line comes in and your home wiring begins. You can unplug the cable (it looks like a phone jack), and plug a phone in here. If you hear line noise, that means it's beyond your house and your phone company will need to fix it. If it's inside your house, then you might have to pay some bucks if there's some bad wiring to fix.

3. If that STILL doesn't work, call your ISP and demand that they send out someone from the phone company or a contractor to check for line noise, they have professional equipment for this and can help figure it out.
Like I said, if it's the phone company's problem, raise hell until they fix it. If it's inside your house, be prepared for it to cost money to fix properly.

Good luck.