State Bank Of India [SBI] — Harassment by telecallers and recovery agents for settlement of wrongly billed credit card dues.

Am writing on behalf of my mother who is a senior citizen staying at Pune. Fraudulent transaction was done by an anonymous person and she received a credit card bill for an amount of Rs 19000, inspite of the fact that she had not done any transaction on the card. When complained to the credit card helpline her card was suspended and she was intimated that she cut the card and dispose it off. After a few days only an amount of Rs. 9840 was credited back to her account. She since May 2020 however kept on receiving payments from various people about payment of balance amount and pressurising her to immediately pay the balance outstanding with interest and some calls even request settlement of dues for a lesser amount. Inspite of mails written to them a standard reply is received stating the following: Due to impact of Covid-19, we are unable to respond to your mail.
2 calls a day at odd hours are received that is causing severe mental harassment to my mother. Would be helpful if you could suggest a way to stop this nonsense. Should I lodge an FIR, complain to BO?
Kindly help