Signal 2.0 is the second version of its free and open source messaging application for iPhone and iPad users. Signal app is specifically designed to make secure and easy-to-use encrypted voice calling. Apple’s iMessage also provides encrypted communication, but there was no way to send secure messages from an iPhone iMessage to an Android phone, or vice versa, unless you signed up for a monthly subscription plan and got the person you wanted to communicate with to sign up for it too. Signal app doesn’t require a login, login, no username, no PIN required.
Source: https://thehackernews.com/2015/03/signal-encrypted-ios-text-calls.html