Get these 2 apps

Redphone - encrypted phone calls
https://play.google.com/store/apps/details?id=org.thoughtcrime.redphone&hl=en

TextSecure - encrypted text messages
https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms&feature=more_from_developer#?t=W251bGwsMSwxLDEwMiwib3JnLnRob3VnaHRjcmltZS5zZWN1cmVzbXMiXQ..

Both apps are by Open WhisperSystems
https://whispersystems.org/