in all honesty, having had first hand experience of ntl/virgin, bt sky and orange broadband/phone services, once things have got to the level your at, only usefull option is to leave, as its far more agro to sort out the mess for them, they wont, than it is to go through the new customer process with another provider.
With that level of incompetance, they cant claim to be suddenly on the ball when it comes to enforceing their contract with you. If that gets their attention you have a direct contact. Think along the lines of... Im quite happy to pay your bill, when you provide the service. No service no bill.
Overseas call centres are of no use, imho they are a
culdesac, a dead end, with no way out as, in my experience, they cant transfer you or communicate outside their own dept... hang up, only thing to do!
I wonder if they are paid by number and length of the calls they recieve?
I also wonder if an advantage, of sorts, of the open reach situation is all providers go directly to open reach for repairs/faults, rather than through bt to open reach. They all have the"£100 fee if the fault is found to be on your property" clause enforced by open reach and, i was told, operators have to enform you of the fee by law.(they cant charge you if you dont let them in by the way, then fix the fault yourself, most people know a bt engineer)