Putting a flip phone's sim card in 3GS with an expired contract

[iShipwrecked]

All the phones in my family are with AT&T. I bought an iPhone 4S (under new contract) after my contract for my 3GS ran out, and my question is whether I can take the sim card from my brother's flip phone, and put it into the old 3GS. (the sim cards look to be exactly the same)

Data and texting has also been blocked on my brothers phone. My hope is that I can just put his sim card in the 3GS and then we'd have an iPhone that could make calls, but not receive data.

Would this work? Or do I need to jailbreak and unlock the 3GS even though it's contract is up?

Thanks in advance

 

[forcetactic]

It will work until ATT randomly adds data plan to your bro's account (could take days, weeks, months).

 

[Applejuiced]

If its on AT&T sim the phone will work. But if he's on a monthly post paid plan they will eventually add the mandatory iPhone data plan to his account. No way around it.
Unless you go prepaid AT&T gophone sim.