You have to search through loads of numbers, but for O2 to giffgaff you have to find 901 and then change it to 443, then copy and save the file to the sd card, then delete the old one and copy/paste the changed file back in to the same location.
Regarding not be able to delete, try ticking or unticking USB debugging in the settings menu. I used rom toolbox pro and I believe I needed to tick it to make changes at root level.
Hope that helps. It does work once its done and it stays changed even if you install a new rom
Any idea what the number would be for Virgin Mobiles voicemail?