Mac Forums

Hi there, I recently bought my friends iPhone 3G from him. I removed the o2 sim card in it and replaced it with my o2 sim card but i get no signal. in the top left all you can see is
_ _ _ _ _ O2 - UK 3G ?
Ive tried restoring all diferent types of settings, tryed jailbreaking it, tried changing sims but when i do that i get .No Service. Please help me?
Thanks