What You Should Know about Signal App
Signal uses your existing phone number and address book. No need to create an account, no passwords to forget. This open-source app is available on Android, iOS, Windows, and Mac.
It is a fast and simple app to send and receive messages. It works well for SMS and MMS, but will also send IMs and group messages. It is free and open-source, enabling anyone to verify its security by auditing the code.