The open nature of VOIP-based calls makes it easy for spammers to send audio-commercials to people’s VoIP voice-mail inboxes in much the same way they invade our e-mail inboxes. The acronym for the problem: SPIT, for spam over Internet telephony.

A “spitter” would harvest phone numbers from a number of different VoIP services, then he would create an audio ad, and send it. Any open, IP-based phone system could be a target: Free World Dialup, SIPPhone, Earthlink’s Free Online Calling program. Other services, such as Skype, AT&T’s Callvantage, Vonage, and Comcast, would be largely immune because portions of those networks operate over a closed system that the SPITters would have to hack.

When this summer, someone tried to send spit to Free World Dialup subscribers, the invasion was thwarted by blocking the IP address of the sender. Now FWD has created a tool called Pulver Communicator that allows FWD users to select people who can call them from a buddy list of sorts.
The application combines elements from social networking programs such as Friendster and IM buddy lists.

So far, there aren’t enough people using open VoIP to justify voice spammers’ efforts to pitch them. But it won’t last, researches predict that by 2008, the U.S. consumer market for VoIP will reach $5.6 billion, up from $320 million this year.

Via Technology Review.