No ads, nags, or in-app purchases. No internet connection required. Fully offline app. This Android app will train you to hear and send Morse code at 20 WPM using an approach very similar to the Koch method. In this app you learn first the vowels, starting with "a", then alphabetically the letters and ordinally the numbers.
You can download the installation package of 20WPM Amateur ham radio Koch CW Morse code trainer 3.0.5 free of charge and read users' reviews on Droid Informer. The app requires Android 2.1 and above. This application is totally secure for downloading. KG9E is the developer of the tool which can be found in the Educational category.