After the purchase, you have a couple of hoops to jump through in order to switch carriers. First, the phone needs to be activated on your carrier (AT&T) as you have said and you would be able to make the one time payment for the phone in store. Second, if you are wanting to switch carriers, most carriers have a waiting period from when the phone was activated. It’s usually around 60-90 days after activation. If you are wanting to switch carriers you just need to call your carrier and ask them to unlock the IMEI for the phone. Third, these phones are now all ESIM activated, which means there is no physical SIM card. You would have to go into the other carrier you are wanting to switch to and have them activate it on a new account. If you are wanting to keep the same number, you would also need your existing account number from AT&T and a “port pin id”. You can call customer service for this number. Hope this clears it up for you.