A SIM lock, confused with PIN or PUK code, is a capability built-in to GSM phones by mobile phone manufacturers. Network providers use this capability to restrict the use of these phones to specific countries and network providers. Currently, phones can be locked to only accept SIM cards from one or more of the following:

• Countries (your phone will work in the US, but not the UK)
• Network/Service providers (e.g. T-Mobile, Orange, etc)
• SIM types (i.e. only specific SIM cards can be used with the phone).

Virtually all mobile phones are shipped with country and network provider locks. In addition, most modern phones have firmware installed on them which is specific to the network provider. If you have a Cingular phone, it displays the Cingular logo and supports only Cingular features. This firmware is installed by the service provider and is separate from the locking mechanism. You can unlock a Cingular phone to work with a T-Mobile network, but the phone will still display the Cingular logo and only support Cingular features. Phones can be unbranded by uploading a different firmware version, a procedure recommended for advanced users only.