Tata Docomo — threat call from customer support to pay bills

I used to use tata services for last 4 years.
I ported to other service provider on 11th of nov 2017, had paid the bills upto 25th oct 2017, tata instead of providing me bill from 25th oct to 11th nov 2017 they prodive bill for full billing cycle, they also generated the bill from nov 25th to dec 24th, during which i had already ported to other service provider, these are fake bills, and now they are threatening me to pay bill to logde complaint, they are harrassing me with continous calls and cheap language, please help