Skype

[Oxonian]

The use of Skype is often mention on this forum, both to contact O2 from overseas and to make free calls over WiFi, particularly from overseas. We even have a forum guide to Skype :-

Guide: A Guide to Skype 

This article gives further hints and tips for using Skype and will help to maximise the Skype user experience :-

I Still Use Skype for Video Calls: Here's Why (howtogeek.com)

https://www.howtogeek.com/i-still-use-skype-for-video-calls-heres-why/