Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
TextMe - TextMe is a free calls and messaging application at both local and international levels.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
FluffyChat - Open. Nonprofit. Cute ?