Private Place is an Android calculator app that hides a secure messenger behind the numbers

The premise is simple enough - you add private contacts and contact them through a built-in secure dialer and messenger that obfuscates calls and messages. The private talk-ulator secures, sweeps, and blocks incoming calls, clears frequent and recent calls lists, and automatically backs up message & call history to Dropbox and Google Drive. As you may expect, Private Place encrypts all its app data, and is also 64-bit compatible, so future-proofing is ensured.

Private Place requires root access, which it uses to prevent Android from shutting it down by mistake. The app is ad-supported, and you're given a two day grace period where you won't be shown adverts. If you like the app, you can pay to remove ads and support the developer.