Signal

“I am regularly impressed with the thought and care put into both the security and the usability of this app. It’s my first choice for an encrypted conversation.”
Bruce Schneier

Signal offers private messaging and calling in one simple app. It is both free and open source. Development is supported by community donations and grants. This means that there are no hidden strings attached. Use Signal as an alternative to WhatsApp or, better still, its replacement.
whispersystems.org