TMobile used to be pretty good, since they merged into EE their customer service has been abysmal, in my experience.My HTC broke and was sent away 3 times consecutively only to be returned each time with the same issue. They told me I had to send it away to HTC and they wouldn’t replace it (even though it was only 5 months old), took god knows how many phone calls and a formal complaint to get a replacement, though they refused to give me another HTC (or any phone other than a Samsung) so I ended up with a Galaxy S4, which I dislike immensely.
Their phone support is poor to average but their in store support is horrific. They have zero interest in solving problems, just selling 24 month contracts. I went in the other day with a problem and the guy couldn’t solve my issue so tried to sell me an iPad instead.