The actual cabling of the phone lines belong to BT Openreach. None of the phone providers, including the Full LLU ones like Sky, actually fix phone faults themselves; they commission OR to do the actual investigation & fixing. So it all boils down to how the phone provider handles the contacts with the user and how efficiently they hand it off to OR.
All I can say is that I have personally found (& I believe RobertoS has said the same) that BT Retail is excellent in its (online) voice fault reporting, monitoring, redirection whilst off-air and informing you of status. They have a target of 3 working days, I believe, to fix voice faults; after which they will pay some compensation.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC