Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
Sipdroid - Free and open-source SIP/VoIP Client
Talk360 - Talk360 is a best-in-class communication utility that offers a convenient way to connect with loved ones globally.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
TextMe - TextMe is a free calls and messaging application at both local and international levels.