When a SIM card is locked, it requires a PUK code to unlock it, which can typically be obtained from the mobile service provider if not included with the user manual. Entering the PUK code incorrectly multiple times can result in the need for a new SIM card. The locking mechanism applies to the SIM card itself, not the phone, meaning that if a SIM card is locked in one device, it cannot be used in any phone until unlocked. Conversely, an unlocked SIM card can be used in different devices. Additionally, some mobile providers may lock phones to only accept their specific SIM cards, limiting compatibility with other networks. For those interested in learning more about cell phone hardware, online resources are available, though specific recommendations were not provided.