help with debrand g502 (uk)

hi i just been given a sony ericsson g502 on the three network (uk) which is unlocked but i have been using the three sim that was in it but the phone crashes a lot and the settings dont work good on othere sims like 02 ect thats why i want to debrand or is it worth just selling it on

can i debrand this phone to get rid off the three network rubbish
and still have the call and end buttons working

and is it possible i can have vodafone firmware or a firmware that will work better on a asda mobile sim which is a vodafone network partner

thank you

ps the firmware is r1fa037