They 'supposedly' contact the box through the phone line on a regular basis to ensure that the 2nd box is indeed in the house as suggested previously.They also want you to have a phone line in order to order movies,extra sports events etc etc to increase profits.
I have sky+ in the living room and the standard box in the bedroom with no landline, so it's not a pre-requisite.
These days they'd be nearly out of business as most people have dumped their landlines.