Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Crisp Chat - Multi-channel customer support software
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FLEX - An in-app debugging and exploration tool for iOS.
Sipdroid - Free and open-source SIP/VoIP Client