If you want to import all your settings into your Jailbroken phone, I’ve created a carrier file… be sure to run ultrasn0w as your iPhone won’t recognize the Sim anyway – the carrier file enters the MMS settings, as well as tethering and 3G settings. It also includes the 3UK Logo for the top of the screeen.

Download it here: 3_custom.ipcc

Edit: If after you’ve installed the Custom Carrier, you can’t send MMS, Goto Settings > General > Network > Cellular Data Network
then in the MMSC box, add http:// before the text that’s in there.

WARNING: If you are using iTunes 9, you’ll need to do the following to enable the Carrier File to be used..

For Mac Users:
1. Close iTunes
2. Start Terminal
3. Run the following command:

defaults write com.apple.iTunes carrier-testing -bool TRUE

4. Press enter, now close Terminal

For PC Users:
1. Run Command Console (Run-Type in Command)
2. Type this command into the console:

“C:\Program Files\iTunes\iTunes.exe”/setPrefint carrier-testing 1

3. Hit Enter

Updating your iPhone with the Carrier file:
1. Open iTunes & connect your iPhone.
2. Click on your iPhone in iTunes(under devices listing).
3. Windows Users: hold shift and click “Check for Update”. MAC OS X: hold alt and click “Check for Update”.
4. Select the .IPCC you downloaded earlier.
5. Reboot your iPhone.