Is there a system to find out what phone number/sim has used my phone? I asked optus this and they said all they can do is block my IMEI. But AFAIK when you put a sim in a phone it registers the sim and the IMEI number of the phone to the network, so I should be able to see what phone number is using my phone and track them down myself. I understand there would be privacy issues but if it is stolen then maybe the police could get involved.