Various service providers and vendors have noticed an increase in VoIP scanning traffic. Arbor Networks mentioned VoIP attacks as one of their increasing concerns. A Norwegian honeynet detected various INVITE requests trying to get VoIP systems on the internet to dial specific numbers. This scan is for open VOIP relays. VoIP attacks are nothing new really and some people in the telco-fraud business seem to have been around for quite a while. What is new is that they are getting detected more and more (and I'm getting more emails about this) which probably means that the scans are on the increase.
Some traffic is borne from custom tools, probably designed from stage one to conduct fraud. Other traffic is generated by publicly available tools such as SIPVicious. My suggestion is to scan your network with SIPVicious, remove any SIP devices that should not be exposed to the internet. If the VoIP system needs to be exposed, at least make sure the the user extension passwords are not predictable (use svcrack to test this).
Here's some blogs and articles that mentioned SIPVicious scans:
drop me an email.