K850i change menu?

I have a o2 K850i but want to put my t mobile card in it works for so long then says invalid sim, is this because of the o2 menus and settings is it possible to flash the phone memoery and install the t-mobile menus??
Help my N95 is driving me crazy...

Thanks

maybe your sim is defective. try another sim. is phone locked to any network?

Probabbly because you phone is locked to one network try debranding the phone.