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
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Sipdroid - Free and open-source SIP/VoIP Client
Facebook Messenger - Facebook Messenger is a faster way to message.