I wrote cingular and this what they said:
for security and verification purposes, all requests for SIM unlock codes must be directed to our Customer Service Department which can be reached by calling 1-866-246-4852 or by dialing 611 on your handset.
Please do not reply to this email, this email address is not monitored.

I was thinking to go to cingular with the mpx200 that why I need a unlock code for it