Lots of companies are getting into VoIP by piggybacking on networks from Volo or Level 3. Few companies are actually building out nationwide networks but I ran into some people that are. CommPartners (CommPartners News) has filed for CLEC certification in 50 states and hopes to be done by summer. They will sell their network and applications as a wholesaler.
They are able to handle calls from IP to IP, IP to TDM or just TDM to TDM. They are targeting resellers that have a customer base of broadband customers. Think ISPs and even traditional VARs.
The company seems well funded and they tell me they think traditional VoIP "marketing companies" aren’t going to be the winners in the long run… you need to own the network to be successful at VoIP. If this turns out to be true, then CommPartners is in a great position.