unlock or debrand

I have a Sony ericsson K800I that i wish to put my o2 sim card in, The previous user had a Virgin sim card in side the phone and was using it fine. I put in my o2 simcard today and i got 'inactive sim' My sim works on my Unlocked Vodafone 360 (Samsung H1) so there are no problems with the sim...

im doing this for my partners sister, and she cannot get her sim card until the phone is done, i need it useable on T-mobile. Thanks.

im doing this for my partners sister, and she cannot get her sim card until the phone is done, i need it useable on T-mobile. Thanks.