Ok, seems to be some myths about what the phone line is actually used for with sky, and sky multiroom, i'll try and clear a couple of these up (i was sky CS for a while, outsourced)
1. you are contractually obliged to keep the box connected to the phone line for the first year, here are the reasons :-
The sky box makes a callback once a month (generally at the same time) and downloads all data of PPV etc, which is then charged to your bill, i have had some stories of people with boxes not connected (or faulty) and people getting 300 quid bill from all the films they have watched for the past 6 months etc.
The line is also used for sky active etc, but does not need to be connected except when its used for obvious reasons, remember a sky dish is one way, the box needs to send info to sky HQ somehow.
2 Sky Multiroom and Sky+, the boxes must be connected at all times or you will incurr the full charge of having each box (£40+ per box on full package), as has been mentioned earlier, there is no proof that these boxes are even in the same house otherwise. if the callbacks are made to sky from different numbers you will be charger, and if one isnt made, you will be charged (lots of problems wtih this one when folks were decorating etc). In the case of Sky +, Sky do reserve the right to remove any recording rights etc.
As for the original post, i can quite believe this, the amount of crappy boxes (and Sky have no idea which you have, its usually just whatever the independent has there, although they can tell through checking serial numbers it doesnt actually state on their system in any clear way which you have) that cause various faults with phone lines buzzing, ADSL going down etc.
It would be fairly easy to find the fault i would have thought though, all you have to do is cross reference the time the exchange goes down, with the callback log on sky's server, although if of course its Sky Active thats causing the issue, in which case it would be remarkably difficult as it could be anyone accessing it at any time. For a fault of this magnitude though, i would have thought the manufacturer of the box would be liable, for distributing a product that interferes with the correct running of a national network.
theres other reasons for the phone line being plugged in too, but Sky do not violate any privacy laws, and the contract you sign is really quite comprehensive and covers all the bases, I can't quite recall all the details mainly because it was a hellish time of my life in many respects and im doing my best to wipe it from my mind.