Inbound and outbound calling

VoIP spam or what is commonly referred to as "SPIT" (Spam over Internet Telephony) has been discussed extensively in the press but the problem appears completely contained. If you do a search on Google for "VoIP spam", all of the top ranked articles are of 2004 vintage. Is it simply that there is far greater economic incentive for a spammer to spend a few seconds to spam millions of people using email than to waste several seconds to annoy just one person using VoIP? If that is the case, voice communications has inherent anti-spamming characteristics.

The technology to implement VoIP spamming is simple. For example, a committed spammer can implement a straight forward plug-in to Skype via the Skype API, and leverage Skype's free SkypeOut service in North America to openly "SPIT" at large numbers of legitimate phone numbers that can be readily harvested from the web. However, this has not happened - for outbound calls from VoIP networks such as Skype to the public telephone network (PSTN).

The new category of service providers that link inbound PSTN calls to the VoIP network such as iSkoot and Rebtel are also potentially susceptible to a different form of VoIP spam. For example, the same spammer can use the SkypeOut service to call blocks of numbers that belong to these service providers thereby connecting the spam calls to unsuspecting VoIP users. In this example, if David on a phone clicks to call Jane on a VoIP network, Jane may receive a call from a spammer instead of David. However, the probability of such a spam attack may be low if the utilization of the inbound circuits is low. Note that a variety of techniques can be used to thwart such spam attacks such as caller ID or the use of access PIN codes. However, such solutions are not perfect as caller ID can be maliciously altered or in many cases the caller ID may be blocked or inadvertently changed by the service provider if the user is roaming. But again, VoIP spam has not happened for inbound calls from the PSTN to VoIP networks.