The next time your mobile phone number affiliated to an online bank account is deactivated especially on a Friday night, you may be in for trouble.

The police say the incidence of online banking frauds, where culprits neutralise bank customers’ mobile phone numbers that receives transaction alerts and then siphon lakhs of rupees from their accounts, is steadily on the rise.

The office of the Commissioner of Police received over five complaints since last December-end from residents who lost amounts ranging from Rs. 1 lakh to Rs. 9 lakh.

M. K. Chandrasekar (31), a resident of Perambur, was a recent complainant. “On Friday evening, March 22, my mobile number got defunct and the next morning I went to a local customer care centre. They issued a fresh SIM card but it got activated only on Sunday night. When I checked my bank account on Monday I found out that a total of Rs. 3.4 lakh was transferred on three occasions,” he said.

“I did not get a single alert message on my phone as I believe the fraudster had blocked my SIM before the transactions. A week before, a man claiming he is an executive from my cellphone service provider called me and confirmed my address, date of birth and mother’s name,” he said.

Investigators of the Central Crime Branch strongly believe Chandrasekar’s mobile number was deactivated by the culprit who could have been the man who called him.

After obtaining Chandrasekar’s bank details by sending him a phishing mail or hacking into the bank’s database or through an insider in the bank, the culprit would have called him to ascertain his mobile phone details.

“In all the previous complaints received, the culprits had gone to a customer care centre, making sure it did not have a CCTV camera. They posed as the victim who lost his phone, gave a fake ID proof and got a fresh card SIM card for the same number issued. He then entered the online bank account of the complainant and transferred large sums as he receives One Time Passwords (OTP) on the mobile phone number in his possession,” said a CCB officer who tracked the accounts to which the money was transferred throughout the country in the previous cases.

One of the first complainants was a resident of Velachery who lost a sum of Rs. 9 lakh. The police tracked the money trail to Mumbai, but hit a dead-end as the receiver accounts were registered with fake identities.

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