Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Twilio - Brings voice and messaging to your web and mobile applications.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Sipdroid - Free and open-source SIP/VoIP Client
Plivo - Plivo simplifies your customer engagement.