RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.