Zoom - Equip your team with tools designed to collaborate, connect, and engage with teammates and customers, no matter where you’re located, all in one platform.
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
WebEx - Cisco WebEx Web Conferencing, Online Meetings, Desktop Sharing, Video Conferencing
Sipdroid - Free and open-source SIP/VoIP Client