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A Project of the Electronic Frontier Foundation 
How to: Use RedPhone (Android)
Download location: https://whispersystems.org/; can also be downloaded from the Google Play store
System requirements: Android 2.2 and up
Version used in this guide: RedPhone 0.9.6
License: GPLv3
Other reading: https://support.whispersystems.org/
Level: Beginner
Time required: 15 minutes
RedPhone is a mobile app for Android that allows users to make encrypted voice calls over a Wi-Fi or data connection using their normal phone number. RedPhone only encrypts calls that are between two RedPhone users, or between users of RedPhone and Signal.
You can choose to make calls to other RedPhone users from inside the app, or you can call someone using your phone’s default system dialer, and RedPhone will automatically prompt you with an option to upgrade to an encrypted call.
How to Install RedPhone
Step 1: Download and install RedPhone
On your Android phone, enter the Google Play store and search for “RedPhone.” Select the app, “RedPhone :: Secure Calls.”
Why can't I download RedPhone without signing up for Google Play?
To prevent tampering or data collection by third parties, it would be better if the Android version of RedPhone was downloadable outside of Google's "Google Play" app store. Unfortunately, for now, these programs use some of Google's infrastructure for software updates and for "push" notifications. You can read more about the creators' reasoning for this decision here.
Select “Install” and accept the Terms of Service by selecting “Accept.” The app will download and install automatically.
Step 2: Register your mobile
Once you have installed the app, open it; you will automatically be prompted to register your mobile number.
Once you have registered your mobile number, RedPhone will send you an SMS code to verify that the number belongs to you. Type in the code when prompted by the app. You have now successfully installed RedPhone and are ready to make encrypted calls!
Using RedPhone
In order to use RedPhone, the person that you are calling must also have RedPhone (or Signal) installed. If you try to call someone using the RedPhone app and they do not have it installed, the app will ask if you would like to invite them to use RedPhone via SMS, but it will not allow you to complete your call from inside the app.
When you make a call to another RedPhone or Signal user (from either your default system dialer or from inside the app), you will be given a random pair of words. This word pair will allow you to verify your identity and keys with the other user—also known as key
verification.
The trustworthiest way to verify the identity of a caller is to use out-of-band authentication to verify the word pair. You can also read the words aloud if you recognize the caller’s voice, although very sophisticated attackers might be able to defeat this if they needed to. The word pair must be identical.