unnecassary bill

i paid my bill on 2 sept 2017 .. and converted my number from postpaid to prepaid..airtel accepted my request on the same day.. my number was off for 2 days as per them .. meanwhile in between they are saying we have used the number and our bill is pending i.e.1000₹ and now they are calling us after 20 days.. first thing i dnt understand how can a bill come of 1000₹ without not using a single call.. i want airtel to please take care of it.. and take strict action against it..