Unlocking sony ericsson 580i

Question

i bought a sony ericsson 580 i from an online auction site and even after inserting a cingular sim card i get the message"insert sim card". people at sony ericsson support tell me the phone is still attached to some service in china. what am i supposed to do??? pleaaaaaaaaaaaaaaaassssssssssssseeeeeee H!E!L!P !!!!!!!