Twilio - Brings voice and messaging to your web and mobile applications.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
CallHippo - Easy to use Phone System with World-Class Support
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.