Users have encountered voice applications before, of course, such as the interactive voice response systems used by airlines to provide basic flight information. The difference, Shapiro says, is that in the past, a company would have had to buy a specialized phone switch and hundreds of thousands of dollars in related equipment to offer such an application. VoIP allows similar applications to be delivered over the network, much as cloud computing services are delivered to companies over the Internet.

For VoIP networks, Shapiro notes, "it used to be that the assumption was that the service didn't have to be as reliable." Ifbyphone was attracted to the FlexNetwork because of Bandwidth.com's promises of a robust network, he says.

The company's claims of reliable nationwide coverage do suggest something "a little unique" for VoIP networks, says Rob Enderle, founder of the research company Enderle Group. "Most VoIP companies provide a centralized service that you can access anyplace, but they're not in the position to ensure quality of service because the network isn't theirs," he says. "If [Bandwidth.com] can actually do that, that would be an advantage for them."

Enderle adds that IP voice networks enable a variety of features that are otherwise prohibitively complex. They lend themselves, for example, to combining voice mail and e-mail, creating a universal in-box for several phone numbers, or adding lots of users to a conference call. (Hardware limitations can make this difficult on traditional systems.) "In terms of convergence," he says, "it makes any of those next-generation things vastly easier to do because you're not trying to juggle an analog network and a data network that for the most part don't work well together."

Missner says the company worked to bridge between different types of networks and technologies to create hybrid features such as the ability to send text messages from phones that are considered landlines, or send text messages via instant messaging programs.

"When you think of voice as an application, there's a lot of innovation that becomes possible," he says. "Having a pure IP voice network is a catalyst."