Another alternative is call the person saying that you are calling from NOKIA and his mobile no is one of the 20 nos that have been selected for prize based on selection of IMEI numbers.
And you would be needing the name, and detailed address along with landmark so that the prize can be couriered to him.
Else call up Airtel helpline and tell them your problem . www.airtel.in
There might be the menu which is tedious but I think you can get thru (use airtel network to make the call so its free)
But, please keep the details of the mobile purchase with you (The original bill and xerox copies just in case, this will have the IMEI number mentioned in it and thus it can be proved that you are the purchaser of the mobile phone) and also police complaint for theft of mobile phone.
Hope this helps.
With best regards