In this case, the scammers let the phone ring once — just enough to cause a missed call message to pop up on victims' phones — and then hang up. Victims who call the number back are connected to a paid "adult entertainment service" located overseas.
http://www.nbcnews.com/technology/missed-call-one-ring-cell-phone-scam-could-cost-you-2D12044967
http://www.nbcnews.com/technology/missed-call-one-ring-cell-phone-scam-could-cost-you-2D12044967