Libon is an app that lets you make international calls to fixed and mobile numbers in any part of the world. Calls are made over IP, so you don't even need a SIM card in your device, although you do need an Internet connection.
To make a call, you'll first need to install the app. After that, you can buy minutes on the app or at a vendor. After that, all you have to do is dial the telephone number you want to call from anywhere in the world. If you want, you can also send credit to another person even if they're in another country, so they can communicate with you.
To pay, you can use your credit card or your Google Play Store balance. Payment requires a minimum number of minutes, after which the corresponding balance is deducted for any additional time. The cost of calling landlines is usually higher than that of calling cell phones.
Calls are charged per second. There are no connection fees or other hidden costs, so the charging system is very flexible. You can call more than 150 countries around the world, although keep in mind that the cost of the call varies depending on the country to which you're making it.