> There is increasing demand for voicecall encryption. Unlike instant messaging, which effectively allowed companies to start from scratch and so has resulted in a number of highly secure products, phonecalls run over older infrastructure and almost always pass through telecom companies, usually in an unencrypted form (although the information may be encrypted while in transit).
As I'm sure you're all aware, Signal from OpenWhisper Systems  is the best answer ATM, folks.
Uses ZRTP which the table shows as ticking all the boxes. Integrates with existing phone/text functionality on your phone, but switches to secure version automatically if the other user is also using Signal.
IME, it just works.
 - https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms&hl=en_GB