Master Reset Code
Sometimes known as the Master Code, this allows you to unlock a handset if you have entered the Security Code wrongly too many times. If you know you have forgotten the Security Code, entering the Master Reset Code instead will do (even though it is probably more digits). You should ask your Service Provider for this code when you need it.

Special Code/Service Provider Lock Code
This code is required to allow a phone locked to a particular network's sim cards to be used with a sim card from another network. Normally, only Orange and T-mobile lock handsets in this way, though O2 and Vodafone are beginning to lock PAYG and WAP handsets.

The unlock code varies from individual handset to handset, and you can only get it from your service provider. Expect to pay around £15 to £35 for this code, plus any line rental up to the end of the initial contract period, except with Virgin, who provide this for free, as long as you have spend £30 on call credit.

Blacklisting
This is not a security code, but it may work like one! To help combat mobile phone thefts, the networks share a a database of the IMEI (serial numbers) of mobile phones that have been reported stolen. Once a phone is on this list, it will not be allowed to use any of the networks.

This can be a risk if you buy a second-hand phone, because if it has been stolen, or if an unscrupulous seller makes an insurance claim after selling, the phone you bought in good faith may be permanently disabled. Be very careful to check the source of a second-hand phone!