Open Whisper Systems' Signal app is no longer limited to keeping text chats out of the wrong hands. A beta version of the Android app now includes experimental support for video and voice calling. Both sides of a conversation will have to switch the features on in settings for this to work, but you're otherwise free to talk knowing that encryption should prevent eavesdropping.
It's not certain when the feature will be available to every Signal user, although the phrasing of the update suggests that it's more a matter of "when" than "if." And iPhone owners won't be left out -- OWS has mentioned that video and voice will be available in an upcoming iOS beta release.