http://packages.sky.com/terms/terms_skytv.aspx
According to this you only need to remain connected to a phone line for 12 months, which is news to me as I thought it was an ongoing requirement. (I believe this is how they send software upgrades, but maybe someone more knowledgeable will have a better answer.)
When I added Sky Multiroom, it took a couple of months to get a phone line connected to the second Sky box and even though I'd been a customer for years prior, I had a couple of letters "reminding" me that the phone line was not connected - so Big Brother IS watching you!