Idea Cellular — Harassment continues even after porting

Address:	122508
Website:	Ideacellular.com

My Mother ported her number from Vodafone postpaid to Idea Prepaid haryana, since Vodafone postpaid guys looted her for 5 months. She cleared all the payments before porting. Now even after porting, Vodafone sent a bill to pay and someone calling from Idea as well to pay previous bill and sending this SMS:
From IZ-Idea
Dear Customer, We are not in receipt of proof of payment made with your previous operator. Hence your number would be barred. Request to kindly produce payment proof in 10 days to avoid disconnection

I would like to know when all the payments were cleared before porting, why now Idea is calling and sending sms saying that this nimber would be barred. Is this even legal?