One of the problems facing the VoIP industry is of course SPIT. SPIT is the SPAM of the Internet Telephony industry. Robot diallers are a huge problem in North America and I have a friend who always listens to who is leaving an answer phone message before picking up the call. A high proportion of calls are from computers.
In the IP world it is even easier to make huge volumes of VoIP calls from a computer, particularly because there is potentially no cost involved. The model here is the same as for SPAM which is of course essentially free of charge.
The interesting dilemma is that whilst a SPAM filter can monitor and email for particular types of content this is not possible in Internet Telephony where a call has to be set up and answered before the callee knows who is speaking to them.
We therefore have to employ more sophisticated techniques in spotting this type of traffic and in general an ITSP will monitor the call traffic on its network to identify unusual patterns. For example if a specific caller is making multiple calls inside an unreasonable short space of time then it cannot be a human making the call. Alternatively if calls to many different end users are going unanswered then this too is unusual behaviour and is likely to be a computer.
The level of SPIT facing an ITSP has not yet reached the proportions of SPAM which can be over 90 percent of all incoming emails (if you are receiving a high level of SPAM you need to change to a professional anti SPAM service). It is however certainly something that a serious ITSP takes seriously.