All of the necessary technologies and communications standards exist today for voice communications that are as secure as OpenPGP email. Both proprietary and open source solutions exist for desktop and mobile devices that implement the necessary bits to provide a solution without dependence upon one global service provider. ostel.me provides both a service and an application for the Android OS that is only marginally more complex to use than dialing an existing phone number, while still based entirely on open standards like SIP and ZRTP. The app is experimental and is based on existing open source client code provided by the CSipSimple, pjsip, and zrtp4pj projects.
With Lee Azzarello