Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Sipdroid - Free and open-source SIP/VoIP Client