Project Tox, also known as Tox, is a FOSS instant messaging
application aimed to replace Skype.
With the rise of government monitoring programs,
Tox provides an easy to use application that allows you to connect
with friends and family without anyone else listening in.
While other big-name services require you to pay for features,
Tox is totally free and comes without advertising

Source: https://github.com/TokTok/c-toxcore