The number of mobile VoIP users is set to grow to 100 million in Europe and the USA in the next two years.

And, according to Juniper Research, a considerable number of these will by-pass existing mobile networks by using Wi-Fi networks – Juniper estimates that through these actions the mobile operators will lose $5 billion of revenue by 2015. Some mobile operators have already reacted to the emergence of mobile VoIP by blocking services like Skype.

The research company has published a study Mobile Voice Strategies: mVoIP Opportunities & Business Models, 2010-2015, suggesting that mobile carriers would have to work with industry partners to maintain market position.

"By 2012 we expect significant uptake of mobile VoIP in its various different flavours", said Anthony Cox, senior analyst at Juniper Research. "By that date mobile VoIP will be available over both 3G and WiFi networks. We also anticipate that several more traditional operators will have joined 3UK and Verizon in the US and developed relationships with mobile VoIP players such as Skype," he added.