Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
Sipdroid - Free and open-source SIP/VoIP Client