Indian police announced at least 70 arrests on Thursday in connection with a massive telephone scam to cheat American taxpayers out of phony “back taxes.”

This was an all-too-familiar scam that hit Americans with recorded messages, purportedly from the Internal Revenue Service, informing the victim that back taxes were due, and a lawsuit was imminent. The victim was encouraged to call back and arrange emergency payments to “settle” the IRS case. Some targets were bilked out of thousands of dollars.

The Associated Press quotes Mumbai police saying the scam “brought in more than $150,000 a day,” which would make the operation worth almost $55 million per year. The Wall Street Journal says the ersatz call centers were making thousands of calls per day.

“Some victims were also told to buy gift vouchers from various companies, and hand over the voucher ID numbers which the impostors then used to make purchases,” the police added.

Up to 600 people could ultimately face charges including “extortion, impersonation and violations of India’s information technology laws” for their role in the operation, which was big enough to fill several stories of an office building in Mumbai. Hundreds of hard drives, high-end servers, and other electronics have been seized.

The gang had accomplices in the United States, who provided personal information on the victims, and took a 30 percent cut of the loot.

“U.S. authorities have struggled to combat an epidemic of swindlers targeting taxpayers. The Treasury Inspector General for Tax Administration, an IRS watchdog, said it has received more than 1.7 million complaints in the last three years from people reporting phone calls from swindlers impersonating IRS agents. More than 8,800 victims have paid more than $47 million as a result of these scams,” the Wall Street Journal reports.

The New York Times states that an inside informant helped to bring the Mumbai operation down after it had been up and running for about a year. Owners and operators of the scam were reportedly “stunned to see the police” because “they never thought that police would raid them.”

The NYT describes how the operation used cell phone text message to frighten Americans into handing over their money:

The call center employees would send mass text messages announcing an urgent inquiry to as many as 10,000 American cellphone numbers, said Parag Manere, deputy commissioner of police in Thane, in the western state of Maharashtra. If recipients called back, the call center employees would introduce themselves as “Christopher or Daniel” and speak with an American accent, impersonating I.R.S. officers. They would then warn the victims that the local police or I.R.S. agents would raid their homes within 30 minutes unless they sent an immediate payment, said Makarand Ranade, another Thane police official.

Another phone scam reported to the IRS involved threatening voice messages. It’s not clear from the New York Times account if the gang sending the text messages was also behind these robocalls, although the report does mention that seven distinct call centers associated with the crime ring were operating out of the same building.