Vodafone India — threatening me that they will take legal action after all bills paid

Address:

Chennai, Tamil Nadu, 600053

I was being followed by vodafone for a mobile bill of amount rs. 252, which is completely fake. I was undertaking the service from voda & ported out to airtel clearing all the previous bills. Now voda is asking again to pay a bill of the said amount and not in the position to clarify of the same. After asking about the bill, they said since i have ported my no., they will not give any bill.
I was just considering it as a fake or ghost calls, but today they have hand delivered me a lawyer's letter with all illegitimate words of cautions.
Now please guide what should i do? The company wants money without any reason (Unable to produce an invoice) & threatening for legal actions for not paying the same