I had a similar experience to IceCreamConewithaflake with BT. And the hours spent on customer service... Anyway, after that, I phoned up to tell them I was changing provider. They set their retention team on me.
They offered me a deal. Nowhere near as good as the other provider.
They offered me faster broadband. Not available in my area.
They offered me BT Sport. Don't want it.
They explained that, yes, they were more expensive but you did get the Customer Service for it. I always try to be polite to the poor people manning the phones as they're not usually the ones that have messed up but I'm afraid I laughed and laughed and laughed and laughed.
At that point they got a bit Bad Cop and threatened that I'd have to pay a charge for leaving the contract early. As it happened, as I knew full well, my contract term was up. BT ended up owing me money. Teehee!
I'd had a previous bad experience with them at an ex-DPs address where they weren't our provider but we did have one of their phones hard wired into the wall rather than connecting via a socket (old property and old phone line). We were redecorating so wanted to get rid of it. Three times, the engineer didn't turn up. On the third occasion, DP failed to get to an event important to me because he was waiting for them. At that point, I lost my shit, yanked the phone out the wall, drop kicked it down the garden and told them to fetch it at their leisure.