I think that the terms and conditions involving you getting a subsidised box in return for the phone line is in your Sky contract. I believe it is referred to as an “Interactive Discount” or “Interactive Subsidy”. I *think* the company who gives you the discount is a subsidiary or sister company of SSSL, the company who supplies you with your Sky subscription 😉
The sky box calls in once a month, usually in the middle of the night, and to a 0800 (UK) / 1800 (Ireland) number. It identifies itself to Sky and then hangs up – from what I remember, the call takes less than a minute.
Of course, once you are out of your 12 month minimum term, you can disconnect the box from the phone and Sky will have no problem with it. The only exception to this is Multiroom – the reasons for the phone line connection is twofold; the discount as above, and of course, to make sure that your “second room” isn’t actually John down the road who just wants a cheap Sky subscription through you!!