The app uses TelephonyService to get your phone number, which is your unique identification on the service.
The app uses your Contact listing to initiate new chats.
No data is stored elsewhere on the app.
It also uses Internet for sending messages, note that although the messages are small (see MQTT) you should always use WiFi when possible to avoid extra charges. I will not take any responsability for extra data charges this app may cause.
Always avoid using ROAMING when connecting to internet if you're unsure about your phone service internet charges.
The source code is freely available at GitHub: https://github.com/agoransson/generic-messenger/
Please note that this app uses MQTT 3.1.1, currently there are no openly available brokers for this protocol, but you can start your own 3.1.1 broker using the testing software found at https://wiki.eclipse.org/Paho/MQTT_Interop_Testing_Day
Update: Mosquitto 1.3.1 is now available and it supports both 3.1 and 3.1.1 MQTT specs.
The application uses the Qatja MQTT client library, available at https://github.com/Qatja/android