Unlocked cell phone help with Alltel and Tmobile Please....?

I have a few questions..

1. Do tmobile 'pay as you go' or 'prepaid' cell phones have sim cards in them or is there just a battery?

2. If there IS a sim card, and find an Alltel cell phone that is unlocked to Tmobile, if i put the sim card from the tmobile pay as you go phone into the unlocked altell cell phone, will it actaully work like this and still be Prepaid?


thanks
ten+