Calcutta police’s cyber lab has stumbled on a type of phishing fraud where criminals first trick telecom service providers into giving them duplicate SIM cards of potential targets before hacking into their bank accounts to transfer funds.

The modus operandi came to light following the arrest of five fraudsters, including a Nigerian, in Mumbai on Monday.

Churchill Ugochukuriigwesez, 25, and his accomplices Shantanu Sarkar, Sima Saha, Ansu Tushardeep Biswas and Amarjeet Yadav would approach telecom service providers by masquerading as customers who had lost their cellphones and acquire duplicate SIM cards. “The idea was to get the original SIM cards blocked so that the targets wouldn’t receive SMS alerts from their banks when the fraudsters transferred funds, using Internet banking details procured through phishing emails,” said Damayanti Sen, the joint commissioner of police (crime).

A phishing mail is one that appears to be sent by a bank to a customer, asking him or her to validate personal details. If someone ignorant about the dangers of phishing responds to such a mail, the senders immediately gain access to their online banking details.

“The one deterrent to surreptitious funds transfer is the text message that banks send to their customers after such financial transactions. So ensuring a target doesn’t get that one crucial message is a masterstroke,” the officer explained.

Complainant Sanjib Chakraborty, a bank employee living in Bhowanipore, recently discovered an online transfer of Rs 2.10 lakh from his account without his knowledge. His cellphone had stopped working a few days before that, but he had no idea it was linked to the bank fraud.

“When Sanjib contacted his service provider about his deactivated SIM card, the customer care division said that was because a duplicate had been issued. By the time everyone realised what had happened, the money was gone from his acccount,” an officer said.

The police arrested Enarul Islam and Hamidul Rahman, the two beneficiaries of the funds transfer, but it turned out that the duo had also been duped. “They were looking for loans and the fraudsters had posed as bank officers and got them to open the accounts,” the officer said.

The gang acquired Sanjib’s bank details when his wife responded to a phishing mail that appeared to have been sent by her husband’s bank. “The mail actually originated from the Nigerian kingpin’s laptop,” the police said.