@OP: If you are talking about IMEI, did you try using another operator's SIM card?
Because, operators can put your IMEI in black list, white list or in grey list. If in case your IMEI is in black list, using another operator's SIM card would solve the issue.
[EDIT] But I see no reason why your IMEI should be blacklisted (saw your other thread where you mentioned that the phone you purchased is not pre-owned) [/EDIT]

EDIT: But I guess, even if your device is blacklisted, it should show the IMEI...