Thieves are targeting cell phone users, particularly teenagers and millennials, with a "ring once, then hang up" scam. Best advice: Don't call back if the number is unfamiliar.

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After the scammers ring and disconnect, curious recipients may view the number and call back -- and those people have been getting slapped with international call charges that start around $20 and go up from there. Some fees have approached another $9 per minute, for each additional minute.

The scam seems to be focusing on a number of teens and millennials who may not leave or listen to voicemails. These scammers also tend not to leave voicemails.

Melissa Companick of the Better Business Bureau of New Jersey said if you get scammed, check with your carrier about erasing the charge.

"That's not always going to be the case but it can't hurt to ask, you know, and say, 'It was just an accident and i didn't want the service,'" Companick said. "The sooner you do that, the better."

The center of the scam appears to be somewhere in the Caribbean. Barbados is one known hot spot for call sources.