Does my iPhone have to be unlocked to use on straight talk?

My service 2 year contract is over and I'm done using AT&T. I want to use straight talk. Does my phone HAVE to be unlocked? I already purchased the sim. Can I just activate it now?

Added (1). On the straight talk website it says that ANY AT&T T-Mobile phone will work.

Yes, it has to be unlocked first.AT&T might do it for free. The only type of phone that doesn't have to be unlocked for Straight Talk is a T-Mobile phone.

Yeah LD

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