Transferring sim to iphone 4?

My friend has a contract with T-Mobile and her handset is roughly 3,000 years old. I want to give her my old iPhone 4 but I'm not sure if we can transfer her contract/number to my old iPhone? Is this possible, and if so, how do we go about it? I was with Vodafone, not sure if that info is needed.

Unless your iphone 4 is unlocked from Vodafone, then she will be able to use it. I doubt that your iphone 4 is unlocked. The majority of phones are locked with their specific carrier and so you won't be able to use it with another cell phone company like T-Mobile.

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