It used to be voice, then there came a host of added value services (that was long time ago). With Internet and OTT we have seen a few voice services (Skype, Viber and several others) coming up, exploiting the IP to funnel voice -VOIP.
More recently we have seen messaging service providers that have decided to piggy back voice on their messages. The last in the row, for now, is Snapchat that is releasing a host of new features to its image messaging software.
Last year Nielsen analysed the way people are using their smartphones and how the use changed over time. It looks like most of the time is spent on Apps (not on calling someone...), 85% in the US. And people are using on average some 26 apps a month. Whilst the time spent using a phone is increasing, and so it is increasing the time spent using apps, the number of apps used tends to remain stable. Even more important, it seems that 84% of the time spent on apps (which is 85% of the total time we use the smart phone) is spent on just 5 of them.
Becoming part of that top 5 is crucial!
This is the main drive leading to the bundling of services within a single app. Each app provider aims at keeping the user within their "garden" and providing voice service is one way of doing it.
The coding of voice in bits has become so effective that just a marginal bandwidth is sufficient to deliver an acceptable quality of the communications. Hence it has become "easy" to funnel voice services in other services. We can expect voice to become a standard add on to most apps, including game apps (where the first drive can be to allow gamers to talk with one another but in reality as a tool to keep the gamer in the app all time).
This means that in a few year voice may disappear (almost) as a stand alone service. We will find it in many of the apps we are using. The crucial point (something that still needs to be worked out) is to find a way to route voice from the calling app to the app being used by the called party. I guess this will be a point that will be addressed by 5G, something that is not under the radar of most 5G researchers but that I feel will become a winning point from a market point of view.
5G, at least to me, will be a revolution not in physical transport layers (1-3 of the ISO stack) but at the higher layers, session, presentation and application.