Can i switch an at&a iphone 4s to tmobile?

My girlfriend gave me her iphone 4s for at&a i was wondering if i could switch it to tmobile without unlocking it or do have to have it unlocked just to switch it?

Have to unlock it first. Best luck.

You'll need the AT&T iPhone 4S to be unlocked before you can use the phone on T-Mobile. To get your iPhone 4S factory unlocked, mail me for references.

Its AT&T & yes you'd need to get it unlocked for it to work on T-Mobile.