Its not uncommon to arrive at work in the morning, fire up your e-mail program and find your inbox littered with spam. Weve become accustomed to the ritual of deleting these pitches. But what if you arrived at work and your voicemail announced that you had 40 new messages–and that 35 of them were unsolicited commercial calls? Listening to and deleting these messages would be more time-consuming than trashing your junk e-mail.

Its a reality that isnt often mentioned when people discuss the nascent voice over Internet Protocol (VoIP) technology that is dramatically transforming the telecommunications industry. But some industry observers are cautioning that the open nature of a VoIP phone call makes it easy for spammers to send audio-commercials to peoples VoIP voice-mail inboxes in much the same way they carpet bomb e-mail inboxes today. Most of the industry leaders and observers I speak with acknowledge the threat of spam over VoIP, but knew of no wide-based industry effort to address it. Thats disconcerting. The only tangible step taken so far, it seems, is to make up a new, unsavory acronym for the problem: SPIT, for spam over Internet telephony.

Heres how SPIT would work: A company interested in sending out unsolicited advertisement could easily harvest phone numbers from a number of different VoIP services. Any open, IP-based phone system could be a target of spitters. That includes such services as Free World Dialup, which now boasts more than 300,000 subscribers; SIPPhone, run by former MP3.com and Linspire CEO Michael Robertson; and new services such as Earthlinks Free Online Calling program. Other services, such as Skype, AT&Ts Callvantage, Vonage, and Comcast, would be largely immune to such attacks because portions of those networks operate over a closed system that the SPITters would have to hack. The spitter then would create an audio ad, and send it to however many members he wanted.

Jeff Pulver, CEO of Free World Dialup (FWD), says someone tried this summer to send spit to his companys subscribers. He was able to thwart the effort, however, by blocking the IP address of the sendersimilar to how e-mail spam is often stopped today. As a result of this attempt to send unsolicited advertising at FWD, Pulver created a new tool called the Pulver Communicator, which will be unveiled in about two weeks. The tool allows FWD users to select people who can call them from a buddy list of sorts. Users can set rules as to who can call them, says Pulver. Its a very smart approach, combining elements from social networking programs such as Friendster and instant message buddy lists. Its the kind of endeavor that others in the industry would do well to replicate. But no one else seems to be talking about the issue much–and this is the time when such efforts should be underway.

FWD and others have only a brief window of time in which to create ways to block voice-spam. The laws of economics are on their sidefor now. Simply put, there arent enough people using open VoIP right now to justify voice spammers efforts to pitch them. Until that critical mass is reached, voice spam likely wont become a serious problem. But VoIP is marching toward widespread adoption; research firm IDC predicts that by 2008, the U.S. consumer market for VoIP will reach $5.6 billion, up from $320 million this year. And as VoIP becomes more commonly used, the window of opportunity for stopping spit will slam shut.

And while spam didnt necessarily hamper e-mails adoption, today it is a very annoying –and expensive–problem. Leaders in the VoIP market need to take advantage of this early-market time to address this issue and figure out ways to stop Internet telephony from drowning in a sea of SPIT. Unfortunately, however, not much is now being done on an industry-wide scale. People should be aware of the voice spam problem, says Pulver. On the Net everyone can be reached, and what people have done with e-mail spam is indicative of what people will do with a voice-based service.