MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
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).
WebEx - Cisco WebEx Web Conferencing, Online Meetings, Desktop Sharing, Video Conferencing
tSIP - tomeko.net/software: SIP client for Windows.
GoToMeeting - GoToMeeting is a web conferencing service offering a range of services which are available on Mac, PC, iOS and Android devices.