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).
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
Sipdroid - Free and open-source SIP/VoIP Client