If you have Multiroom (ie more than 1 Sky box in different rooms) then you have a "mirror" subscription - the same channels as on your main box are available on the second (and third) box. You get this at a cheap rate of £10 per extra box, rather than 2 or 3 times your normal subscription cost.
Now, you could give this 2nd box (and the cheap £10 cost) to someone else at a different address - thus depriving Sky of revenue. To make sure you don't then the multiroom subscription Terms and Conditions require that ALL boxes on the multiroom sub are connected to the SAME phone line ALL THE TIME (none of this "only for 12 months" malaraky - it's ALL THE TIME for multiroom). The boxes dial "home" to Sky every so often and thus confirm what phone number they are connected to/dialling from via CLI. If this doesn't happen they write to you to tell you to sort it or they will charge you full price subscription for the 2nd (and 3rd) box!
They take no prisoners and take no excuses on this - it is policed VERY thoroughly! You will only get (I believe) a maximum of 3 letters, then they just bill you the full sub for all the boxes.