Basically I signed up for AT&T home internet, and it DID NOT WORK, period. It was unusably slow and cut out constantly. I couldn't use it. I told them so within a week, they sent someone to fix it, they failed to fix it, I told them I'm canceling. I proceeded to return their router and switch to a different provider and immediately got fine service.
But AT&T didn't cancel my service. They said I couldn't because I was locked in for a year or some arbitrary time period. It's been five years now so some details escape me-- but the key thing is that after some months they claimed I owned them $600, and I said I wasn't paying when I never got anything from them, end of story, goodbye.
My bank took their side on the part of the bill that had already been paid for that first month. I explained what had happened, and the bank "talked to AT&T" who no doubt showed them a sheet of paper with the words "He signed up for our internet and we installed it," and my bank said there was nothing to do. So that first month was a sunk cost.
But I never paid that $600. Simple as that. They forwarded it to a collections agency, I ignored it. I told them, "don't call me: I will never pay you."
It went on my credit report and my credit dropped about 40 points, making no appreciable difference to me. I ignored it. Over time the credit report recovered, and somehow after about four years AT&T withdrew the whole thing without any further action from me. It's no longer on my credit.
Moral of the story: if you can withstand a small dent in your credit, just tell MEGACORPXYZ to fuck off.
Sad that this is what you have to do these days, but it's better than no options at all.