OK to clarify everything a bit
There is no technical requirement to have you Sky+ box continuously connected to a phone line, it will need to be connected to one when you first turn it on though so it can callback to sky and activate your card, after that it only uses the phone line for interactive services and for box office movies so as long as you don't use these it will work without being connected to a phone line.
However by not having it connected to a phone line you are breaking your terms of use with sky and they may suspend your viewing or even send somebody out to uninstall your system. with sky+ this condition exists for the whole time you have the box, not just the 12months I first thought.
it differs from normal sky where the phone line condition is for the first 12months only and also with normal sky you can pay and opt out of this condition, you can't with sky+