Skype
Skype is a telecommunications application software product that specializes in providing video chat and voice calls between computers, tablets, mobile devices, the Xbox One console, and smartwatches via the Internet and to regular telephones.
FaceTime
FaceTime is a proprietary videotelephony product developed by Apple Inc. FaceTime is available on supported iOS mobile devices and Macintosh computers that run Mac OS X 10.6.6 and later. FaceTime supports any iOS device with a forward-facing camera and any Macintosh computer equipped with a FaceTime Camera.
WhatsApp Messenger is a freeware and cross-platform messaging and Voice over IP (VoIP) service owned by Facebook.[44] The application allows the sending of text messages and voice calls, as well as video calls, images and other media, documents, and user location.