We have sky multi room and have both boxes connected to our landline as per terms and conditions. However, we got the nasty letters about the boxes not being connected to the landline even though they were. It turns out that if your phone number is ex-directory then they cannot tell if your box is replying to a callback as the number does not show up. It took a few irrate phonecalls from yours truly to put a stop to the letters. That was about 2 years ago and i've heard no more about it.
Hope that's useful to someone.
Hope that's useful to someone.