Virtual phone number specialist Voxbone has sold a majority stake to private equity firm Vitruvian Partners and is planning a program of accelerated expansion and product development with the help of its new backer.

Brussels-based Voxbone SA provides wholesale international VoIP services in 56 countries to many of the world's major communications service providers and large enterprises, including Orange Business Services , Deutsche Telekom AG (NYSE: DT), Telecom Italia Sparkle , NTT Communications Corp. (NYSE: NTT), Telefónica , Level 3 Communications Inc. (NYSE: LVLT) and Skype Ltd. It has built a global network of IP points of presence (PoPs) and developed a homogeneous SIP-based service that conforms to multiple regulatory regimes. (See Orange's Libon Goes International via Voxbone and NTT Taps Voxbone for Enterprise VoIP.)

While being very protective of its financials -- the company declined to disclose its revenues or provide details of the stake sale to Vitruvian Partners -- Voxbone says it is profitable and has experienced average year-on-year revenue growth of 29% since its inception in 2005. Its only external funding has been €1.1 million (US$1.2 million) in 2007 from angel investors, including Emerge (also based in Brussels).

"We have been self-funded ever since," says Voxbone CEO Itay Rosenfeld. "Then this year the investors had a good opportunity to make a very nice return, so they took it."

Now Rosenfeld is planning greater and faster expansion with the help of Vitruvian. "We have been focused on telephony, specifically in-bound, making sure we are compliant with regulations and building an automated, virtualized service. Recently we have been expanding into new services, such as emergency dialing, driven by demand from our customers, but we have only had the resources to work on a few new developments at a time. Now we will be able to hire more people and work on more service developments simultaneously... we have a long list of demands from customers, and we'll be able to work at an accelerated pace," says the CEO.

Voxbone currently has about 95 staff members, with between 20 and 30 in R&D. Rosenfeld says he's not sure how many he will have in a year's time, as a detailed strategy has yet to be finalized with Vitruvian.

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Accelerated development could also come from targeted acquisitions and partnerships. "We hope we will be able to acquire and strike strategic partnerships. We have 850 customers and we want to broaden the scope of the services we can offer to them, and we can do this more quickly through partnerships and acquisitions," says the CEO, who is looking for small, software-focused companies with cloud and open source experience, especially those "that specialize in real-time media," with WebRTC of particular interest. (See Voxbone Tweaks Its Network for WebRTC.)

Already in development within the existing Voxbone team, though, is the "virtualization of mobile numbers, so they can be embedded in any application." Rosenfeld says, "That's a much harder thing to do than virtualizing geographic numbers. There are many companies looking at this with different approaches… no one can say that they have the right approach. It's very early and very exciting."

— Ray Le Maistre, , Editor-in-Chief, Light Reading