Twilio - Brings voice and messaging to your web and mobile applications.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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
CallHippo - Easy to use Phone System with World-Class Support
Sipdroid - Free and open-source SIP/VoIP Client