Signal is the first free application for the iPhone that scrambles voice calls. It would thwart government surveillance or eavesdropping by hackers. Signal adds to a growing number of mobile encryption offerings from software vendors. Signal displays two words on a screen during a call, which are meant to be verified with the party on the other end to ensure a man-in-the-middle attack isn’t underway. The beauty of Signal is its simplicity. It displays only the contact details of the other user who has it installed.”]
Source: https://www.csoonline.com/article/2459503/iphone-gets-first-free-app-for-encrypting-voice-calls.html