sim problems

Hi all,
I have a samsung galaxy S. I changed from vodafone to o2 but my new sim card won't let me make or recieve  phonecalls or texts. I can make emergency calls and can connect & use 3g but my phone keeps showing no mobile networks. If I put the simcard in another mobile it works perfect, my phone is unlocked for any network and even if I put my vodafone sim back in it won't work now, does the same as the new o2 sim. Anyone got any ideas what the problem is, I have done a factory reset but that didn't fix the problem.

Thanks for any advice or help