e700 network problem

Ok, heres the problem. My phone is unlocked and when I put in my o2 sim card there are always signal bars, but also a handset symbol with a cross through it, and it displays "Limited Service." I know what you're thinking. I've tried reflashing and changing the imei and nothing seems to help. BUT, when the signal bar is full, it works 50% of the time, so im completely stumped as to what to do. Please help!

p.s. the phone seems to work better with an orange sim card, but this is no use!!