In late September, a woman in National City, California, received a voice message on her phone saying she was in trouble with the Internal Revenue Service (IRS) over "tax evasion or tax fraud".
Panicking, she rang the number and told a man who said he was from the IRS: "I can pay $500," half the sum demanded. "I could do a payment plan. I just can't pay all of it at once."
"Ma'am, you can pay $500 today itself. You can do that?" the man asked, adding that lawyers would look at her accounts and work out a monthly payment plan, but she had to pay half now.
In transcripts of the conversation that investigators shared with Reuters, the man told her to keep the phone line open and drive to a nearby grocery store, where she bought $500 worth of iTunes gift cards and gave the 'agent' the redemption codes.
She had just been scammed - one of at least 15,000 people the US Justice Department says lost more than $300 million in an "enormous and complex fraud" running since 2013. The department last month brought grand jury charges against 56 people in India and the United States for "telefraud" scams run from fake call centres in India.
Investigators have arrested 20 people in the United States, and Indian authorities have made 75 arrests following October raids on three premises in the Thane suburb of Mumbai. Charges include conspiracy to commit identity theft, impersonation of an officer of the United States, wire fraud and money laundering.
Indian police say they are looking for Sagar Thakkar, a man in his early 30s also known as Shaggy, who they believe masterminded the scam. Thakkar was also among those named by the US Department of Justice. Reuters was unable to contact Thakkar for comment; he is not known to have a lawyer, and police believe he fled to Dubai last month.
"We are trying to complete the procedure to issue a red corner notice for Thakkar," Parag Manere, a deputy commissioner at Thane police, told Reuters, referring to an Interpol arrest warrant.
Police said Thakkar led a lavish lifestyle, frequenting 5-star hotels and driving expensive cars with proceeds from the scam. He gave one, a 25 million rupee ($365,000) Audi R8, to his girlfriend. "We have seized an Audi car, and are trying to find other assets of Thakkar," Manere said.
The FBI, which is involved in the investigations, declined to comment. The Department of Justice did not respond to requests for comment for this article. At a news conference last month, Assistant Attorney General Leslie Caldwell said the US would seek the extradition of suspects in India, and warned others engaged in similar schemes they could face jail terms.
In interviews before the US charges were filed, police, suspects and call centre workers in India told Reuters how the scam was run.
Training materials and taped conversations, which investigators believe were made by call centre instructors for training purposes, shed some light on an operation aimed to exploit the aged and gullible.
"TAUGHT TO BE TOUGH"
"The revenue was unpredictable. Some days were good, some were bad," Haider Ali Ayub Mansuri, who said he managed operations at one fake call centre, told Reuters as he was returned to jail in India last month after a court extended his custody. He is among the 75 arrested by Indian police.
"On a good day, we extracted as much as $20,000 from a single U.S. citizen," he said.
In India, the sheer scale of the operation surprised many.
For months, hundreds of young men and women worked nights at several call centres in Thane. Callers posed as IRS officers and threatened their victims, often newly-arrived immigrants and the elderly, into paying fictitious tax penalties electronically - sometimes by buying gift cards and turning over the redemption codes, Indian investigators said.
"They used to blast out pre-recorded messages to thousands of citizens who were asked to call back. When they called back, there was a centre just like this," said Manere at Thane police.
Acting on a tip-off, police raided premises in early October as call centre workers settled in for their shift. The buildings housed seven call centres, and over a few days more than 700 people were detained. Most have since been released, but told not to leave the city.
Callers bullied their victims with the threat of arrest, jail, seized homes and confiscated passports.
"There was one instance where an old lady was crying," because she didn't have the money to pay, said a former call centre worker who spoke only on condition of anonymity. "But we kept insisting on the money. We were taught to be tough," he told Reuters.
On a follow-up raid in Ahmedabad, 500 kms (310 miles) north of Mumbai, police uncovered what they believe was "a nerve centre for these centres," said Manere. "A lot of money has been transacted. It's been going on for a few years."
The police raids found little in the way of documentation, beyond some training materials. Another former worker said this was likely because call centre managers stopped employees from bringing pens and phones to work.
Reuters was unable to independently confirm the accounts provided by call centre workers.
WEEKLY INCENTIVES
Another former worker, an economics graduate, told Reuters she took a job without knowing what the centre did. The 12,000 rupee ($180) monthly salary was well below the going rate for a graduate, she said, but it was a job, and "people aren't hiring."
She said several of her colleagues looked as though they had just left high school.
Her first week was spent in training with floor managers. While callers spoke to their victims, she said dozens of trainees squeezed in around the room, and had to memorise pages of dialogue for use on calls.
Another former employee said his instructors told him his work was illegal, but there was "nothing to worry about."
Callers made "fast money", another former caller told Reuters. In comments confirmed by investigating officer Mukund Hatote, the worker said: "For every dollar you brought in, you were given 2 rupees (around a third of a US cent)."
People wanted to leave rather than be involved in something they suspected was illegal, he said, but carried on because managers offered weekly incentives, such as cash or gadgets, for meeting their targets.
