my sisters want to buy new cellphones but they dont like any that tmobile offer. so i was wondering if they could buy a cell phone from a different network and itll still work without any problems? i know sprint doesnt have sim cards but i dont know about the other ones. so can someone please help thanks!

No unfortunately, you cannot use a tmobile sim card in an at&t phone or any other phone that has a sim card. If they were to purchase an unlocked phone, then the sim card could be inserted, but the thing with unlocked phones is sometimes you have bad luck with them. The phone will work just fine, but most times you can't access the internet for ringtones and stuff, it really depends. I honestly wouldn't advise it. Its more of a pain the neck then it is worth.

Im not sure about the T mobile thing, If you are a current AT&T Subscriber, you prob cant use it with a T Mobile Phone, And it Def will not work with Verizon, Verizon is on a CDMA Network, As to where T Mobile and AT&T Are on GSM