Switching carriers taking iPhone and number
Switching carriers from Att to another carrier taking number and phone. How do they unlock your phone and will all my text, photos , contacts etc be saved
iPhone 8 Plus, iOS 15
Question marked as
You will need to ask AT&T to unlock your phone before it can be used with a different carrier.
How to unlock your iPhone for use with a different carrier - Apple Support
Your data should be fine with the exception of any SMS texts. SMS is a carrier function and you may loose those.