CloudTalk - Work locally, grow globally
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
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Sipdroid - Free and open-source SIP/VoIP Client