A new report by Infonetics Research shows that sorting through mobile VoIP options won't be simple anytime soon, even as user numbers explode. It starts with the fact that there are two main methods of delivering VoIP to mobile devices. One is over-the-top (OTT), in which the voice calls travel over carrier-provided data networks. The other is via LTE (long-term evolution), a technology that lets carriers themselves deliver voice calls over IP links to the handset. Prominent OTT providers include Skype, fring, Line2, Nimbuzz, Rebtel, Truphone, Viber, Vopium and others. Verizon Wireless will introduce native mobile VoIP over LTE in 2012.