Just a month or so after I bought it, my T-Mobile prepaid phone started rebooting constantly.
I took it back to the place where I bought it. Naturally, they would not replace it. I got a long spiel about how I had to force it to use one specific network or it would reboot, but how that was not a persistent setting so I'd have to reset it each time I powered up the phone. I found this unacceptable, but had to get pretty enraged in the store before the staff would help me. Ultimately all they really did was get on the phone to customer service for me. I had to ship it back to a T-Mobile service center at my expense. The only option available for a prepaid phone was to go without my phone. (That stinks, but it is unfortunately what is laid out in the warranty paperwork).
About two weeks later, I got back what appears to be a new phone. It has slightly revised hardware, although it has the same model number. I'm happy to report that so far it seems to be working much better.