Wirelessly posted (8520 (126.96.36.1991))
The IMEI is an industry standard identification for GSM mobile phones (use sim cards). You can find it out by going to Options/Status or by dialling *#06#. I find it strange for the phone to work with an orange sim and not an O2 one. It might be worth wiping the device and reinstalling the OS. If you need guidance on how to do this feel free to ask.