Recommended by Edward Snowden apparently Signal Messenger is available free on the Google Playstore. There's also a version for iOS.

 

Both caller and receiver do need to have the app installed though which is the case for most of them I believe.

 

It might be useful for peeps in Thailand to make it a little more difficult to prevent Big Brother from looking over your shoulder.

 

Installation instructions and tutorial: https://ssd.eff.org/en/module/how-use-signal-android

 

Nothing is secure, certainly not in Thailand. The "authorities" simply confiscate the phone(s), pry any passwords from the users and then follow the trail, no need to decrypt anything.

 

That said, both LINE and WhatsApp! have encryption; LINE (Letter Sealing) needs to be enabled by both parties while WhatsApp! has end-to-end encryption enabled by default.