A carrier lock is a restriction that limits a phone to a particular mobile network, often because it was sold under a carrier agreement. The device may reject another provider’s SIM or eSIM until the original carrier officially unlocks it. Unlocking rules and eligibility vary by carrier and region.
For buyers, especially those purchasing refurbished phones, checking lock status helps confirm the device will work with their preferred network. In cyber security, a carrier lock is not itself a security vulnerability, but unclear device status can create practical risks: a phone may be unusable with another carrier or remain subject to account or activation requirements. Verify the lock status with the seller or carrier before purchase, and use official unlocking channels rather than untrusted services.