How to make encrypted call using android device?

"There is no security on this earth; there is only opportunity." Are you often engage in million dollars deal, if your answer is yes then Beware eavesdropping attackers and opportunistic are sniffing your call! The Prism surveillance scandal has shown that the NSA and other agencies can tap into most communication channels.

In such a case, security is paramount and for that, RedPhone an open source app Licensed under the GPLv3: http://www.gnu.org/licenses/gpl-3.0.html
is the perfect app for you. You can download the source code from GitHub and can audit and compile it at any time.

The tools is created by smartphone Security Company and co-founded by Moxie Marlinspike, called Whisper Systems.

This application provides end to end encryption for your calls. It uses a Protocol called ZRTP to encrypt your voice which then is transferred via voice over IP.

How to use?

Redphone is easy to use. You simply install it and get a Redphone account based on your mobile number. There is an integrated dialer and contact manager in the RedPhone app. Whenever you receive a call, it is encrypted through Redphone servers which totally bypass your carrier.

You can download RedPhone from here.

Does RedPhone really means no one can track your call?

I would like to recommend looking through the source yourself, rather than relying on strangers. However I just go through this article “Architecture Overview” for RedPhone and realized the answer for above question is NO, the software's authors and current server operators that is RedPhone themselves can know who you're calling. But doesn’t that mean by installing the application on your phone, you're already giving it access to your contacts, full internet access, your call information, your logs, your local storage, and recording your calls.

How could we find if we could trust the authors then? We want a VOIP-app where there is nothing logged?

Write your own application and run your own servers. It is as Svetlana says - if you trust nobody, your only option is to do everything yourself - including writing your own compiler from scratch, in machine code, on a processor you built yourself.