Swap iPhone SIMs, what happens?

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@jatay wrote:

Thanks, Max69. What is the voice plan associated with -- a particular phone, particular phone number, or particular SIM? The context for these questions: The 4s has a new two-year contract, the contract of the 3GS is about to expire. I want to swap the phone number of the 4S with the phone number of the 3GS without sticking the 3GS with the new contract.

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the contract and commitment should stay with the phone number that was upgraded no matter which phone it is on

111-111-1111 = Iphone 4s w/ contract

222-222-2222 = Iphone 3GS

after

111-111-1111 = Iphone 3gs w contract on numebr

222-222-2222 = Iphone 4S

1.You can't just swap the SIMs. The 3GS uses a full size SIM and the 4s uses the new micro-SIM. You can go to an AT&T store and get new SIM cards, they may or may not charge you for them, usually not. The phone number follows the SIM.

2.There is no contract on the phone, it's on the voice plan. The expiration dates won't change.

Thanks, Max69. What is the voice plan associated with -- a particular phone, particular phone number, or particular SIM? The context for these questions: The 4s has a new two-year contract, the contract of the 3GS is about to expire. I want to swap the phone number of the 4S with the phone number of the 3GS without sticking the 3GS with the new contract.