Airtel GSM
Locking a valid imei handset

I have purchased a BYOND by500 handset with a valid IMEI number and bill. I have verified the IMEI with airtel 121 service it says its valid, apart from airtel 121 service i have confirmed the validity from 57886, 53232 and www.numberingplans.com all have confirmed my handset to be valid. but airtel has stopped accepting my handset since 30 nov 00:00 hrs.

And as a matter of fact i tried using other service provider in this handset and they are perfectly working.

MY handset: Byond BY500

IMEI :358873024060012

What should i do?