Why must I connect my Sky Digibox to the phone line?
Check your contract. If you have signed a contract that says you must connect your Digibox to your phone line for 12 months, then you ought to (but you might get away with unplugging it for most of the time). The reason for the phone connection is that the consortium (which includes BT) hopes to make money from you when you order a pizza, movie or vote for something. You can waive the phone requirement by paying an additional £25.
However, if you have signed a "multiroom" contract to get an additional viewing card then you MUST keep both Sky Digiboxes connected to the same phone line, permanently. Failure to do so will result in a letter from Sky. If you ignore it, they'll charge you the full subscription price for the second card.