How do you transfer audio and video from device A to device B (and viceversa)?

I would like to create a system that allows a station A to communicate via audio/video with a geographically distant device B, via telephone network
A simple 1 to 1 real-time video conference
If one of the two devices was a Windows PC it would be even better
Can you give me some guidance on which components to use and how to proceed?

https://community-appinventor-mit-edu.ezproxy.canberra.edu.au/search?q=Agora