Vodafone — Constantly being called and harassed by some unknown number saying I have to pay the remaining bill

I am being followed up by some guy whose number shows on my phone as wrong number and constantly asks me to pay the Vodafone-Idea remaining bill i.e after I ported my number to Airtel. when i ask him what is the bill about, he tells me its the last bill generated after i ported - GST and something.
He is telling the services were not immediately shut down after i ported and hence the bill. Now i dont understand if the company didn't shut down the services immediately then how is that my fault ???? why am i being charged for that. when i ported to airtel around 2 months ago i paid all the pending bill, why should i pay the bill generated after my porting ???????????
Can somebody explain me.