Whisper Systems announces the release of Signal, an app which allows to make free, worldwide, encrypted voice calls for iPhone, and that is "fully compatible with RedPhone for Android."
Signal uses your existing number, doesn’t require a password, and leverages privacy-preserving contact discovery to immediately display which of your contacts are reachable with Signal. Under the hood, it uses ZRTP, a well-tested protocol for secure voice communication.
Signal was designed specifically for mobile devices, using a jitter buffer tuned to the characteristics of mobile networks, and using push notifications to preserve battery life while still remaining responsive. Signal is also Free and Open Source Software, allowing anyone to audit the code for correctness or help contribute improvements. The project even pays out a percentage of donated Bitcoin for every merged pull request.