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Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Slack with your Users - Speak to your app's users entirely within Slack
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Conversity - Easiest way to manage Facebook Messenger chats
Sipdroid - Free and open-source SIP/VoIP Client
Avochato - Text messaging for your business.