A Carrier Lock (or SimLock) is a software restriction set by a cellular provider that ties an iPhone's baseband modem to a specific mobile network. The device will rigidly reject SIM cards or eSIM profiles from competing providers until the original contract or financing obligations are fully met.
This restriction acts as a digital fence strictly around the cellular modem, not the entire operating system. If you insert an unsupported SIM card, the baseband simply refuses the cellular handshake and displays a "SIM Not Supported" error, but the rest of the phone (Wi-Fi, apps, camera) continues to function perfectly. Importantly, a phone can be network-unlocked but still restricted by an iCloud Activation Lock, meaning buyers must check both statuses separately.
