I had problems with my line (constant hum) soon after it went live in February and had an engineer visit.
On the BT website, where one goes through the fault reporting, is a section about the customer accepting the possibility of a charge. You have to tick this box or cannot get an engineer visit.
Engineer came, heard the hum, acknowledged it was audible and checked I had nothing else plugged in. (He then sat in his van and had his lunch.)
Came back with his kit, unpacked laptop and some tester unit which reported a fault (I will have to check if I have noted the name of the device he had). Anyway, he says he is going off to check from the nearby cabinet (green cabinet around the corner, which holds lots of wiring links).
Comes back 30-45 minutes later and says that the hum is something I will just have to accept, and that I'm being a bit fussy over it. Does not say he considers no fault to be present.
2 months on and I am billed 125 pounds. Complain online to BT (amount due to leave account in a few days which will either be turned down by bank, or cause me to go 100+ overdrawn, and HSBC will charge me the same {ie 100+} as a 'fee'.
BT rang me back, and chap says he cannot consider the amount to be "in dispute" because I acknowledged there may be a charge. Also fobs off the delay for it reaching account as problem with BT Openreach (the engineers).
Seems to me (a) since one has to tick the box to even get an engineer, it is unfair to then claim that I cannot dispute the amount, and (b) since BT billing sees BT Openreach as being a different organisation, they have to make it a dispute between me and BT Openreach (how convenient - BT's billing team then points the finger at Openreach as the villains!)
So in my view, you are 'stuffed' if you want to dispute a bill caused by an engineer visit, due to sharp practice with this 'tick the box' situation - no tick, no visit, no fix or tick and BT says "you cannot dispute our bill".
Gah! Just wanted a chance to vent anger and frustration, as the hum is the least of the problems - some weeks have had no incoming or outgoing (no dial tone) calls and no broadband for up to 5 days. If the phone doesn't ring, I don't automatically think "phone line is dead again" but that's been happening, apparently. Will have to get usage data from ISP to show I was not connected for days at a time to identify exactly how many days over past 2 months...
(With a mobile phone and mobile dongle for laptop, it has sometimes gone completely unnoticed until someone said she got some BT message about a fault "all weekend". Sod's law that I missed her call in particular!