(CBS) -- "I don't think my adrenaline has ever been that high."
The kidnapping calls came to a father in the Dallas area…
"He told me if I dropped this call, he was gonna kill my son".
Kidnappers called Valerie Sobel in Los Angeles too, saying they'd taken her daughter…
"The man says,'We have cut off her finger and if you want the rest of her in a body bag, it's your choice," Sobel said.
To get her back, he told Sobel, she'd have to send them money. And in the meantime..?
"If you disconnect this phone, she's dead."
She stayed on the line for over two hours, and sent close to $4,000 dollars to Mexico as directed. Then, the would-be kidnappers hung up.
But her daughter hadn't been kidnapped… she was fine. But the FBI says these fake kidnapping calls are on the rise.
"It's a get rich quick scheme where they can extort victims and have them wire money into Mexico," said Tim Fergusen.
Ferguson is an FBI assistant section chief who says those calls usually come from criminals in Mexican prisons. He says prisoners used smuggled cell phones to random-dial numbers in the U.S. and make their threats. Then, prisoners' friends or family pick up the cash sent by victims to money transfer locations.
"I don't necessarily think that it's just in the prison systems, but I would say the large majority of the ones that we do have do come from the prison systems because the individuals there don't have anything but time."
So how do you know if the call is a fake? The FBI says out-of-state area codes are one tipoff.
And the request to wire smaller amounts of money from multiple locations, down to Mexico.