Airtel — Prepaid plan- not able to make calls to 1860 numbers

Address: Indore

I have a prepaid unlimited call plan on my airtel number. Whenever I am trying to call any customer care number starting with 1860 for any company/bank, it says that YOU HAVE INSUFFICIENT BALANCE TO MAKE THIS CALL. what does this mean? I have paid money in advance and the company says unlimited calling, then why not connecting this call.