Vodafone India — vodafone asking money without bill, after ported out

Address:	Gautam Buddh Nagar, Uttar Pradesh, 201301
Website:	www.vodafone.in

I was being followed by vodafone for a mobile bill of amount rs. 1072, which is completely fake. I was undertaking the service from voda & ported out to airtel cellular clearing all the previous bills as said by vodafone store employee at sector 50 noida. 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 lea-gal actions for not paying the same.