Now I might be talking bullshit here, but....
My understanding has always been that the phoneline is only ever used for your Sky box to dial out, to use such things as the piss poor "interactive" features on there, some games, shopping, email etc. These calls are usually chargeable (maybe "premium") so it's another income stream for them. Your box never "receives" an incoming call.
When they send a signal to your box to change something, update something, activate your porn package

ect, that comes in via the satellite signal.
They don't like to let you have a box that isn't connected to a phone line in case they miss out on the 23p that the call generates during the 5 seconds when you're shouting "f*ck it's dialling, quick backup, BACKUP!!!!", but in reality the box will function perfectly well without the phone line.
Actually, my Sky box is now connected to my broadband, so there's no reason at all why it should be connected to a phone line.... yeah, I'm off to unplug the phone line right now!!
Hope that helps
