Vodafone — VICare - Unbilled amount being charged after porting out

I have ported my connection from back then Vodafone to Airtel 3+ years ago.
Recently I have been receiving messages from ViCare mentioning that I have unbilled amount outstanding during the porting out process. I have also received a warning that if I do not clear the amount of Rs. 489.92 then it Vi will create an interruption with my current network connection according to the TRAI guidelines.
Before porting out I had cleared all my dues with Vodafone at the service center itself. Post clearance received from Vodafone I had done the porting out process.

Why suddenly after 3 years ViCare remembers that there is a due amount under my number?
And why does the amount unbilled?
Please provide proof of of the so called unbilled amount.