Mesibo Messenger is an open-source app with real-time messaging voice and video call features. The entire source code this application is available on GitHub https://github.com/mesibo/messenger-app-android

Features
One-on-one messaging and Group chat
High quality voice and video calling
Rich messaging (text picture video audio other files)
Encryption
Location sharing
Message status and typing indicators
Online status (presence) and real-time profile update
Push notifications
Latest versions are also available through Google Play Store and Apple App Store:

Source code:
https://github.com/mesibo/messenger-app-android

Build Instructions and Tutorial:
https://mesibo.com/documentation/tutorials/