Mahanagar Telephone Nigam [MTNL] — Someone using my fathers Mobile number fraudulently

Address:

Navi Mumbai

My fathers mobile number [protected] is somebody else using fraudulently for whatsapp. How it is possible that any other person using my father's in use mobile number, Unless and until you issue his number to other person. How and why you issue my father's in use mobile number to other person.
On 5th February 2022 afternoon I have recharged my father's mobile number for extention of validity service for one year. And this day at night only I have come to know about this fraudulently using my father's mobile number for whatsapp.
So please check for this issue and resolve as soon as possible