Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Twilio - Brings voice and messaging to your web and mobile applications.
Sipdroid - Free and open-source SIP/VoIP Client
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.