Customers can obtain a Virgin Mobile unlocking code for qualified devices and accounts by calling Customer Care at 1-888-322-1122. Unlocking generally refers to the phone being able to be used on other carrier networks, but not all phones will be able to do this even after the unlock code has been used.
Either the MSL or the SIM card can be unlocked. The MSL, also called the master subsidy lock, is the manufacturer's lock on the phone, and disabling this can cause the phone to become completely inoperable. The SIM card generally controls what network the phone operates on, and unlocking it can allow users to take their phones with them when they switch to other carriers.