Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
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.
tSIP - tomeko.net/software: SIP client for Windows.
Facebook Messenger - Facebook Messenger is a faster way to message.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).