On one hand I mean classic stuff from provider's perspective, like malevolent user setting up a premium number in some remote country (Afganistan, Nigeria, etc.) with insane rates (5$/minute and more), which he is calling for long periods. This is often completely automated with multiple simultaneous calls on multiple premium numbers and callee is actually an IVR. And you can't touch the user, because you've promised him unlimited calls. Such schemes can be even more elaborate with many 'proxy' providers in between, so you don't even know, what is the real cost of the call until after the fact (you receive a giant bill from some remote provider). I've seen examples of voip providers losing thousands of dollars a day and in one case even going under, just because they were unable to respond quickly to fraud.

On the other hand I would like to point out the user's perspective. Recently I've read about a case where a blind person calls a lot. Really a lot. Like from the evening to the morning, many days a week, domestic and international calls. It's his form of communication, since he rarely travels abroad or meets with friends in person. So to reduce his monthly bill, he went to a provider that promised unlimited calls. Needless to say, after a few months they've disconnected his number and billed him for premature end of contract (it is customary to sign 2-year contract), citing small print which mentions fair-use.