The iPhones will work if u never pay monthly bills. They just stay locked carrier. Thats why there alot of locked att iphones on amazon. If u check their imei it will show u its locked and if u try to do unlock request through att website it will show u it can not be unlocked cuz money owed on iphone. Iphone will remain working under only to the carriers it owe money too. And their sister carriers like att also owns cricket so if u buy locked att iphone it will also work with cricket carrier. Same goes with tmobile. Verizon and such. Theres a way to get carrier locked iPhones unlocked through 3rd party or u can do it urself on laptop its called SimUnlock but if u ever restart or hard reset the iphone the sim unlock becomes locked again till program reinstalled. Which is annoying cuz everytime u update ios iphone requires restart.