avaamo - Simple, Secure Business Messaging
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
SupportKit - Embed Zendesk support into your iOS app with 1 line of code
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Fleep for Business - Simple, open collaboration and chat with admin capabilities
Sipdroid - Free and open-source SIP/VoIP Client