One of the many questions on here is about PAC or PUK codes, but there is a lot of confusion over which code is actually needed.
So, here is a (very) short guide about them.
A PAC code is a Port Authorisation Code. It is the code you need to give if you want to transfer your number from one network to another (say moving from Vodafone to o2).
A PUK code is an Pin UnblocKing code. You need this if you have entered the wrong PIN code too many times and you have locked your SIM card. This code will not unlock your phone to allow you to use a SIM from a different network. If that is what you are after, you will need to follow the below guide:
You only have so many attempts to enter a PUK code. Get it wrong too often (normally 3 times) and you will permantly lock your phone. Please be careful!
If you have any questions then please post on here - but remember, this is a public forum so do not post any personal details (contact number, home address, etc), and we are not customer services so anything account specific we will not be able to help.