Just go to AT&T, get the phone and let them know you want to keep you phone number. They can transfer the number.
https://www.fcc.gov/consumers/guides/porting-keeping-your-phone-number-when-you-change-providers
Just sign up and request to keep your phone number. By law, you are allowed to keep your phone number.
https://www.fcc.gov/consumers/guides/porting-keeping-your-phone-number-when-you-change-providers