The above link is a very thorough review of the Hello Talk application for iOS and Android. The app is free for a single language, and I installed it based on a language learner’s recommendation but have yet to try it, to experience its complete range of potential.
As a smartphone app, I am skeptical as to how this application can compete with Skype, since a large part of my language partnering involves sharing files and Internet links. Most of my mentoring in English involves uploading files, directing the learner to self-study methods available on the Internet, or connecting them to the exact location of a grammar explanation. A single Skype session with me, often results in a week of homework, and the other person is very thankful for my thorough direction. I am not sure I can do that with a smartphone app, but I’ll let you know if it is possible if I personally use the product.