The best place to get these settings is from your service provider. Call up on their customer care hotline and ask. They'll send an SMS containing the settings.
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
