My computer is not directly plugged into the phone line and wasn't at my previous address a few months ago, but why does it now disconnect when you are using the phone? It is broadband and they don't tell you this!
What is it plugged into? Can you describe how you're set up? Are you using one of those wee filters - my phone and broadband both go into that and it's the filter that gets plugged into the phone socket.
To add to stevie21's answer, all the phone sockets in the house need a microfilter. This splits the signal needed for ADSL and for voice communications. If you don't have a microfilter, using the phone could interfere with the broadband signal and cause it to disconnect. You also might be able to hear the ADSL signal interfering with voice....