Users of mobile video calling services such as Skype are forecast to increase to over 130 million by 2018, but question marks remain over the potential for mobile video calling to generate revenues in its own right, according to Juniper Research. The company observed that as with many OTT services, only a small proportion of mobile video calling users pay for the service directly. OTT mobile video calling players are therefore widening their service offering and opening their client bases to third parties or developing their own apps in a bid to increase monetisation options. Meanwhile mobile operators and rival OTT carriers are collaborating on creating advanced "carrier OTT" voice services, potentially representing the most important change in the voice market since the advent of VoIP itself. Report