A carrier lock, which is featured on most cellphones, is a software code placed on a phone that prevents its use on another network. To unlock the technology, another code must be inserted on the phone. Devices that do not have the lock or code specifically advertise or state they are unlocked. Google, Android, Nexus and Apple feature unlocked devices.
CNET further explains how locking and unlocking affects cellphone use. Typically, locked devices operate on the GSM network. Service for the phones is provisioned with an integrated circuit known as a Subscriber Identity Module, or SIM card. If the device is unlocked, the SIM card just needs to be removed and a new SIM card added by the new carrier. Companies such as T-Mobile and AT&T are part of the GSM network and offer this feature for their customers.
On a CDMA network, phones do not feature SIM cards. Therefore, the current carrier needs to be contacted to reprovision the technology. Carriers such as Sprint and Verizon Wireless are part of the CDMA network. However, neither of the carriers are willing to reprovision devices that are made to be used on the other's network. As a result, a Verizon phone cannot be reprovisioned for Sprint use and vice versa. A GSM phone offers more flexibility in this regard.
According to PC Magazine, CDMA is an acronym that stands for Code Division Multiple Access, while GSM is short for Global System for Mobiles. Both networks represent the two primary radio cellphone systems in the United States, as of 2014. Most of the United States operates on the CDMA network. However, worldwide, GSM is the preferred network for wireless technologies.