SIM CARD — SIM card is "Unregisterred "

Address:

Coimbatore, Tamil Nadu

I usnig an AIRTEL sim corresponding to number: [protected] (issued on 06/2008).
But this SIM is now showing a message like "Unregisterred SIM"after inserting it in mobile. I tried this in all the GSM modbile but the same problem exist.
I call Airtel customer care representative, but they are saying this number is assigned to some one else. But if I am trying to call this number it says "Mobile is switched off."

I as really very much surprized-> " How this number can be assigned to some one else without even informing me". This number is with all my friends, the person assigned with this numbr may get disturbed. I have done some 50Rs recharge for this number also. But all the things gone in vain.
Also, all my vendors of my company ha this number only to all. So, plesae tell me how can I proceed further..
I am realy very unhappy from the response from the customer care. They did not give me any solution.

This type of action by a sim service provider is really shameless.