It seems many, like me, have been struggling to change the carrier logo on iPhone 4's using the T-Mobile UK network. Now I think I know why the conventional methods don't work.

Even with no theming applied my phone has seemingly never used the Default and FSO .pngs in the T-Mobile_UK carrier bundle. I'm sure of this as the phone always only displays "T-Mobile" whereas the stock .pngs in the carrier bundle show as "T-Mobile UK". I have also noticed that since changing my system fonts the carrier name font changed too.

Also, quite recently, T-Mob UK merged with Orange UK so now T-Mob users can roam on the Orange network (2G only). When the phone picks up the Orange network the logo changes to "T-Mobile Orange".

So all this suggests that somehow the OS must be overriding the carrier bundle and writing the carrier name directly in the status bar, perhaps picking it up from the SIM card.

I've searched through so many .plist files but so far cannot find anything to link the OS to the SIM card