Sky (including Sky+ SkyHD) with no Multiroom doesn’t *require* a phone line to be connected (but you pay an additional install fee of €35(ish) ). You can avoid that additional fee by agreeing to connect the Sky box to the phone line for 1 year.

Sky with Multiroom requires all boxes be connected to the same phone line (at all times, not just the 1st year), so that Sky can make sure all boxes on a given subscription are in the same building and you haven’t loaned them out to friends, neighbours, etc.