People normally link their banking account number with mobile phone sim card and after demonetization, online money transactions have increased rapidly. Because people have no cyber security awareness so they fall victim in the hands of scamsters.
How Scamsters Operate-
Scamster calls his victim claiming that he is from his mobile service provider and says that you are using 3G sim card and company is offering you 4G sim free of cost for improved internet services. He then sends 20 digit number by SMS after disconnecting the call. Victim sends this 20 digit number back to the caller or tells over phone and presses options 1 or 2 as asked by the caller. Within few seconds victim's sim card get deactivated and this is what scamster wants.
Victim thinks that his mobile services will resume within sometime unfortunately sim card never gets activated. And scamster do online shopping using his linked bank account.
Actually scamsters already have a new duplicate sim for the victim's mobile number, which they get either by producing fake documents or by impersonating the victim. After getting the duplicate sim card, scamster calls the victim and uses new duplicate sim card to receive OTP from the bank..