Rebtel App Review: Cheap international calls on Android

Rebtel is an Android/iOS app that allows users to make international calls at cheap prices. Cheaper than you would actually have to pay via your own carrier to call someone in another country. This is probably why it is also quite popular given that Google Play alone shows between 1-5 million downloads (and I reckon it is towards the upper limit) and on their website they mention handling more than 1 billion minutes of calls each year.