OneSignal

Signal is a cross-platform centralized encrypted instant messaging service developed by the Signal Technology Foundation and Signal Messenger LLC. Users can send one-to-one and group messages, which can include files, voice notes, images and videos. It can also be used to make one-to-one and group voice and video calls, and the Android version can optionally function as an SMS app.Signal uses standard cellular telephone numbers as identifiers and secures all communications to other Signal users with end-to-end encryption.