I saw Elon Musk tweet about this awhile back. Looks like Snowden uses it as well.
Signal allows users to send texts, videos, audio or picture messages with end-to-end encryption, just as they would via a normal text message. "Signal’s advanced privacy-preserving technology is always enabled, so you can focus on sharing the moments that matter with the people who matter to you," the app writes in its description.
Say "hello" to a different messaging experience. An unexpected focus on privacy, combined with all of the features you expect.