The following apps allow you to make free worldwide calls to other people that have the same app installed. Security wise, not only are your calls encrypted, additionally, VoIP apps bypass data retention laws, calls made with a calling app are not recorded by your network provider.
The best apps to make a secure call are those that are open source, available for Android and iPhone and encrypt your call with keys you only hold, you should also try to go with a company that does not have servers or offices in a country where mass surveillance is known to take place.
RedPhone: Free worldwide end to end encrypted calls implementing the open standard ZRTP, app source code is open to review, there is no need for another ID, this app will use your everyday phone number to make and receive secure calls.
Simlar: Developed in Germany, open source app to establish end to end encrypted calls with the ZRTP protocol, a cryptographic key agreement for VoIP calls, not even Simlar developers can listen in to the calls. To protect from man-in-the-middle attacks, Simlar shows a small code on the screen that can be read to your contact to confirm that you are both looking at the same.
Wiper: This app keeps no logs of the encrypted calls, it can also be used to send messages with a wipe option that will erase them from the wiper server, your phone and your friend’s handset, making later recovery impossible. An integrated Bitcoin wallet in Wiper lets you receive or send payments without leaving the encrypted chat, transactions will show on the same screen.
Zoiper: Encrypted VoIP calls with TLS/SRTP and ZRTP protocols, this softphone can be used in Windows and Linux desktop computers as well as mobile phones, it appears to be targeting the business market. The program can be used in a call centre, hooking up remote workers with a business telephone system.
CoverMe: Full mobile phone communications suite securing calls, messages, files and phone storage. CoverMe encrypts your calls, sends selfdestructing text messages and creates an encrypted vault in your phone where to store private data. A decoy password can be set up in the event that you are forced you to reveal it, the app also assigns you a US phone number that can be used to receive calls.
CryptTalk (subscription): Peer to peer encrypted calls using standard algorithms and perfect forward secrecy without any server involved in the process, only communicating parties have access to the encryptions keys, third party decryption of text messages is not possible. A monthly subscription is required to use this service after the trial period but you can use it for free indefinitely in receiver mode.
Signal: Compatible with the Redphone app in Android from the same developers. Signal is open source, using ZRTP for secure voice communications. Calling somebody who has not installed Signal will be trigger an SMS link prompting them to download it. The company plans to add secure text messaging that will be compatible with TextSecure and a future release of an Android version.