This article claims that spam will start overrunning VoIP service the way it has with e-mail:

One growing concern is the possibility that unwanted VoIP advertising messages — often referred to as spam over Internet telephony, or SPIT — will overwhelm IP voice systems, much as email spam has overwhelmed email messaging systems. Over the years, spam has reduced much of the functionality of email — dozens, hundreds, and even thousands of unwanted email messages clogging up servers and users’ inboxes to the point of driving many users away from email entirely.

While SPIT hasn’t yet become a major problem, experts anticipate that it could do so in the future, as the increasing popularity of VoIP makes it a more attractive target. Let’s look at the potential misuses of VoIP and discuss how you can fight back.