A carrier-locked phone is a device that’s restricted to work on a single wireless carrier’s network, meaning it can’t be used with another carrier’s SIM card until it’s officially unlocked.
In the past, locked phones also tied customers to specific networks, such as GSM or CDMA, which further limited switching. GSM phones relied on removable SIM cards and were commonly used by carriers like AT&T and T-Mobile, while CDMA networks, used by carriers like Verizon, required phones to be tightly integrated with the carrier's infrastructure. This technical divide made changing carriers inconvenient or impossible without buying a new device.
Today, carriers often lock phones to protect the subsidies and financing deals offered to customers who are switching phone companies, ensuring customers stay on their network long enough to recoup the cost of the discounted devices.