Being a Senior 1st Line Engineer for a Business ISP, all ADSL/VDSL Carriers (TalkTalk, Sky, Plusnet, Zen etc.) rely on Openreach infrastructure which is Cabling, Exchange and NODE. LLU providers such as Sky and TalkTalk have their unbundling equipment in the Exchange, but when Sky unbundled exchanges all new DSLAM's and Frames were installed by Sky so you effectively could say that their Network is the newest as far as Exchange equipment is concerned.
After the Exchange the Carriers are relying on Openreach or Virgin Media Backhaul circuits to deliver the traffic to the Access NODEs where the connection hits the Carrier Network.
After that you are on the public internet.
Where you have NDT or No Dial Tone there is clearly either a break, imbalance or earth/battery fault on the circuit which is purely down to Openreach's infrastructure. In this case, nothing to do with Sky.
Shawrey.