Like everyone else, I don't see the justification for Skype valuation in the billions. It's technology and service is not compelling enough and there are technical and quality issues. And it's biggest asset, the user base, is a sandcastle: it will get washed away at the next big wave of free higher quality services.
I think the really valuable real estate in the VOIP market is not the VOIP service but the VOIP directory and routing services. VOIP directory service enables me to make and receive calls regardless of which VOIP service I am using. VOIP routing service lets me easily switch device and route calls.
To get that started, I think the easiest path is to use real world phone numbers as VOIP endpoints instead of using VOIP service-specifc handles like Skype ID. Endpoint claims and disputes are resolved using phone-based authentication.