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.