The codes tend to be unique to the phone. Here, almost all phones are subsidized phones, and locked to a carrier (usually tied to a 2 year contract). Usually, by contacting the carrier, you can acquire the code to remove the sim lock, if the contract timeframe has expired. This prevents people from getting a subsidized phone (often as cheep as free) from one carrier, canceling service, and using the phone with a sim from another GSM carrier.