The threat is coming from smartphones and from the internet. The 27C3 (The 27th Chaos Communication Congress) shows us a great presentation about the threats that are coming from mobile phones.
Mobile phone networks could be swamped by text messages to phones in a denial of service attack by hackers, academics warn. A paper by Enck, Traynor, McDaniel, and La Porta of Pennsylvania State University explains that if too many text messages are sent to phones in the the same cell of a mobile network at the same time the cell's control channel might be monopolised, preventing new calls from being initiated.
"Moreover, attacks targeting the entire United States are feasible with resources available to medium-sized zombie networks," the Exploiting Open Functionality in SMS-Capable Cellular Networks paper (PDF) states.