Vodafone India — Payment due

Address:

Bangalore, Karnataka, 560037

I had a vodafone postpaid connection from July'15 to December'15 after which i ported it to airtel. before porting i had cleared all my outstanding dues. but even after that I used to get repeated calls saying that 780 is due.
Yesterday I got a mail which stated that I should make payment immediately in order to avoid further action.
Payment done in order to avoid hassles. Each time I had requested for an itemized bill which Vodafone refused to provide me. So this is the reason why i had not made the payment. I still do not know why I had to pay 780 even after clearing all my dues. This is not expected of vodafone.