How to Unlock a Network Locked Phone

Through the Network
1. Find your phone's IMEI number. This is often colloquially referred to as a 'serial number,' and is a unique 15-digit number which is found on the label underneath your phone's battery. You can also find this by typing '*#06#' into your handset. This is one of the many codes that performs operations on your phone, and is used in the same way as the unlock code you'll receive later. Copy your IMEI number down onto a piece of paper.
2. Contact your network. Most networks will provide you with your unlock code in exchange for an often relatively large fee. If you have been with them for a while, they may be merciful, and give you the code for free, but in most cases they will charge.
3. Give them your IMEI number. This is needed to generate your unique unlock code and is the only information they will require besides the make and model of your phone. Networks are generally quite slow in sending out your code, and you may be waiting around a week to receive it.
4. Type your unlock code into your handset. This can normally be done without special preparation, but some phones require you to switch it on without the SIM in to remove the network block. You will see a message confirming that the block has been lifted.
Through Independent Companies
5. Get your phone's IMEI number in the way described above (type '*#06#' into the handset). Many mobile phone users need their phone unlocked at some point, and as a result, there are many companies online, such as The Phone Unlockers, that provide unlock code for a small fee.
6. Enter details of your make and model into the website, and quote your IMEI number in the space requested. Websites often charge a lot less and deliver a lot more quickly than the networks themselves, and are in many ways the better option. You will usually have your code within a few days.
7. Input the unlock code into your handset. Your phone will now be free from the network block, and you can change networks. Unlocking also increases the resale value of your phone.