MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Zoho Meeting - Free video conferencing, online meeting, desktop sharing solution from Zoho.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Airtime - Hang out with group video, share music & videos
tSIP - tomeko.net/software: SIP client for Windows.
Voxox CloudPhone - Get a local or toll-free business number that automatically answers, routes and transfers calls, plus other professional business phone services.