Are you planning to offer an external API for Viber? There are many obvious benefits to this:
* Using "centralised" messaging and voip apps gives a consistent user experience regardless of the service being used. Anything non-standard scares the hell out of civilian users (like my parents)
* It could be properly integrated into Android messaging, and threaded with regular sms. My recipients are guaranteed to read it, instead of disregarding yet another popup from some random app.
* Most importantly, many unnecessary steps and other ugliness could be automated or removed by third party access.
(I'm reminded of this everytime I see that annoying "This contact does not have Viber yet" dialogue ) The basic fallback to standard services used by Viber won't cut it when other non-standard voip and messaging services are also being used.
In essence, I want to use one single app that will automatically pick the best available option, and do this silently and robustly.