Phone Line Problems..?

I currently do not use my landline for phone service, I use it for DSL. I am getting disconnected alot, its random. I have hooked the line for the modem directly into the phone companys box outside and still it disconnects, then reconnects randomly. When it disconnects and doesn't reconnect quickly I have hooked a phone to the line and found that there is a busy signal. Have had the phone company out twice and they say the line is fine and that it had to be the modem. I have a new modem and still having the same problems. Any help would be appreciated... Thanks..