avaamo - Simple, Secure Business Messaging
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
SupportKit - Embed Zendesk support into your iOS app with 1 line of code
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Fleep for Business - Simple, open collaboration and chat with admin capabilities
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.