I have been unable to find a single reference for truly tracing a VOIP number. From my experience with VOIP this makes perfect sense to me. While it is certainly possible to trace the IP address and even location of a visher if ISPs cooperate, the DID, if it exists, may very well not be the same as the caller ID being asserted (SpoofCard, n.d.).
Wang et al (2005) have done some very interesting work tracking 'anonymous' VOIP traffic through encrypted networks. By 'watermarking' packets with latency signatures they were able to identify and correlate peer to peer VOIP calls. While this solution is non-trivial and may not apply to every situation where one might be trying to trace a visher, if sufficient law enforcement interest could be generated in the case its application is a possibility.
Also noteworthy in this domain is work like 8els's (2009) to get real tracing working with VOIP. While their application is the 999 system in England and seems to require the aforementioned Telco/ISP cooperation at this time, it's still a good test case and may help get of the some legislation required to foster said cooperation on the books.
It may eventually be possible to get governments or NGOs interested in Cao et al's methodology (2003) in combination with or instead of some of the above mentioned efforts. But currently there doesn't seem to be any consistent, reliable way to trace a VOIP phone number. For the time being it seems we must hope for the cooperation of ISPs and/or governments to track visher's IP addresses, which, given the globally distributed nature of the Internet, may sometimes be a complicated long shot.
8el (2009) New system to trace VOIP emergency calls [Online]. Available from http://www.8el.com/news/voip-news/trace-voip-emergency-calls-19224639.html (Accessed: 26 June, 2010)
Cao, F., Ha, B., Padmanabhan, R., Yuan, A., Tran, K. (2003) Call Filtering and Tracking in VOIP Telephony [Online]. Abstract Available from: http://md1.csa.com/partners/viewrecord.php?collection=TRD&recid=200407330257CI&q=&uid=789600165 (Accessed: 27 June, 2010)
SpoofCard (n.d.) Caller ID Spoofing, Voice Changing & Call Recording [Online]. Available from: http://www.spoofcard.com/ (Accessed: 26 June, 2010)
Wang, X., Chen, S. & Jajodia, S. (2005) Tracking Anonymous Peer to Peer VOIP Calls on the Internet [Online]. Available from: citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.64 (Accessed: 26 June, 2010)