Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Smooch.io for WhatsApp - Engage with your customers on WhatsApp like never before
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
WhatsApp for LiveChat - Let your customers reach you via Whatsapp
Sipdroid - Free and open-source SIP/VoIP Client
EfficientChat - EfficientChat is a completely functional CRM & customer operation API based on WhatsApp.