The Apple iPhone is tied into O2 and the iTunes App Store in the UK. However, for as long as the phone has been on the market, industrious programmers have worked hard to bypass these software lock-ins and free up the iPhone to third-party software and non-O2 sim cards.

The two-stage process to freeing up your iPhone is called 'jailbreaking' and 'unlocking'.

Jailbreaking

Jailbreaking involves downloading an application to your iPhone that frees up the handset's file system to allow third-party applications to be installed that can offer services that Apple or O2 may not. For instance, a jailbroken iPhone can easily be used as a tethered 3G modem - a standard iPhone needs to pay O2 a monthly fee to do the same thing.

Unlocking

Unlocking is the next stage beyond jailbreaking. This allows you to use non-O2 sim cards on your iPhone 3G - effectively breaking the O2 monopoly.

Stage 1 - Jailbreak your iPhone 3G

1. Make sure you have connected your iPhone 3G to your PC or Mac and updated your iTunes software to the latest version. Then check your iPhone OS is updated to the latest version (3.01)
2. Create a new folder on your Mac or PC desktop - call it 'jailbreak' or similar, the name doesn't really matter
3. Download Redsn0w 0.8 for Windowsor Mac OS to the folder you've just created and unzip it
4. Launch redsn0w and browse to Original iPhone firmware OS 3.0. You can download the .ipsw file from here or locate it on your Hard Drive. On a Mac it can be found at /Macintosh HD/Users/Your User Name/Library/iTunes/iPhone Software Updates/
   Remember: you need to point it at the 3.0 version not 3.01
5. Redsn0w will verify the file and if it's valid will start to patch it with the jailbreak code
6. You will then be given some options to install: select Install Cydia on its own
7. Now comes the fiddly bit - you have to put your iPhone into the 'DFU mode'. First make sure your iPhone 3G is switched off and plugged into your computer. Then click on Next
8. Then hold down the Power button for 3 seconds
9. Keep holding the Power button, but also hold down the Home button for 10 seconds
10. Release the Power button but keeping holding Home for a further 30 seconds. If you've got it right, the app should say: "Please wait while your device is being jailbroken. Waiting for reboot..." If you don't see that message repeat the last three steps to successfully get the iPhone 3G into the DFU mode
11. Your iPhone 3G will now be jailbroken and reboot - the process can take up to 15 minutes, so be patient. When it has finished the app will show the message "Done!" and the iPhone will restart
12. When your iPhone has restarted you will see the application Cydia on your Home screen. Your iPhone has been successfully jailbroken

Stage 2 - Unlock your iPhone

1. Launch Cydia on your jailbroken iPhone 3G
2. Click on the Manage icon at the bottom of the screen
3. Click on Sources and then Edit
4. Click on the Add button at the top of the screen and enter the address 'http://epo666.ultrasn0w.com' (Note the zero in sn0w)
5. When ultrasn0w has finished downloading, click on Install
6. After ultrasn0w has installed turn off your iPhone 3G and switch the SIM to the one you want to use. Reboot your iPhone and it should now be unlocked and usable on a different carrier