Is it possible to get a pre-used iPhone on a Tmobile plan?

My phone is broken and my friend has offered to give me her old iPhone 4, my mother thinks it's impossible to get an at&t phone on our tmobile plan, I don't think that's true? And if we do that will it cost any money? Will I get a new number or sim card?

It should be possible as long as you can get the iPhone 4 unlocked. There's a big chance you will get a new sim card because the size of the sim card is probably different. IPhone 4 uses Micro Sim card, so beware of that. You should be able to keep your phone number.