Kotak Mahindra Bank — loan harassment after 17 years which my dad took

Address:

Visakhapatnam, Andhra Pradesh

Hi,
I have been receiving calls from advocate lakshmi stating there is a case being filed on your dad for not clearing the loan which is taken from citi finance in 2003 for 13, 000/-.
I am 10 years at the said time, i don't know how they got my number and i am devastated by their words that my father didn't clear the debt. When i spoke to my father who is disabled, he has informed me that he has cleared the debt with citi finance through agents and however didn't have the receipts now as it is more than 17 years. Since there's no correspondence from them, he thought the loan is cleared and settlement letter is not taken as there's no office to collect it from.
But now i'm getting calls from this advocate asking me to pay 14, 000/- immediately in half an hour or else threatening to follow a case and send notice.
I'm really confused on what to do and why to clear the amount when it's bank's fault for not identifying agent's fraud and they should've informed my dad within that span so that everything would've been settled then and there.
But now how can someone call their son and ask about a loan which is cleared before 15 years and ask them to clear the debt. When i am asking about the details of branch manager to whom i can speak about this, they are not providing at all.