Track VoIP devices in network

you mean IPPhones ? or voice gateways ... or ?

Ciscoworks got tracking utility for IP Phones, with Cisco Call Managers

What do you mean by VOIP devices on WAN networks?

Nmap is a scanning tool that can do OS fingerprinting. Depending on the size of your network you could scan the entire IP block you use and try to ID each device. Presumably all devices should be fingerprinted as Windows, Cisco, Unix, linux, etc. If you get one that doesn't seem familiar then you might want to investigate further.

IMHO I would get written permission from someone in your company who has that authority before doing any kind of scanning.

Depends on the "VoIP device" What is the goal? Weed out rogue users?

You can look for various port numbers, but different systems use different port numbers. Skype is esepcially clever about moving to different ports.

There are professional solutions that seek out VoIP users, such as