Signal

Signal is a free and open source software application for Android, iOS, and Desktop that employs end-to-end encryption, allowing users to send end-to-end encrypted group, text, picture, and audio & video messages, and have encrypted phone conversations between Signal users. Although Signal uses telephone numbers as contacts, encrypted calls and messages actually use your data connection; therefore both parties to the conversation must have Internet access on their mobile devices. Due to this, Signal users don’t incur SMS and MMS fees for these types of conversations. On Android, Signal can replace your default text messaging application, so within Signal it is still possible to send unencrypted SMS messages. Unencrypted SMS messages go through your mobile plan and may incur fees as set by your plan.