Growing ThreatRecently, two hackers rerouted 500,000 calls from 15 VoIP phone providers and billed them to Net2Phone, a small provider in Newark, N.J., a unit of IDT ( IDT - Get Report) by probing weak spots in these networks to steal calls. Cyber crooks can easily steal data, says Graham. "They pick specific 'exploits' -- say, transactions with Social Security numbers or a banking transaction -- and they scan very widely. They go after the vulnerability in the network, not the person." "Vulnerability" is hardly the word that Vonage, eBay's ( EBAY - Get Report) Skype Technologies, AT&T and other phone providers want to hear as they initiate, or expand, VoIP services. VoIP usage is growing rapidly, making it unlikely that hacker attacks will go away. By the end of 2010, research firm Telegeography estimates that nearly 25 million customers will use phone services that run over high-speed Internet connections. The same features that make VoIP attractive to consumers -- it's easy to use -- makes it valuable to criminals, too.