Popular Apple iPhone that is bought through a carrier or reseller and comes with a 2-year contract is locked to one network. Best Unlocked Phones 2017. Samsung Galaxy 6 Unlocked Phone. For example, if you buy iPhone from AT&T American network your device will get signal only through AT&T SIM card. Unlock Avaya Phone. If you try to insert Verizon, Sprint, O2, Vodafone or any other SIM there will be no signal because the phone is locked to AT&T and accepts its SIMs only. AT&T iPhone unlock service is designed to free users from their original carrier. It becomes useful when you wish to switch carriers or need to travel abroad where you don’t want to pay for roaming.