Rob from Frugal Radio has recently uploaded part three in his airband monitoring series. The the last video Rob discussed what communications can be received from the airport. In this weeks video Rob explains how air traffic control is handled over large areas, and also shows examples of what you might hear when an aircraft communicates with it's parent company.
Air Traffic Control Centres provide ATC coverage across most of the globe. This video gives an overview of how you can listen to pilots and Air Traffic Controllers as along their routes.
This video talks about the en route Air Traffic Control systems in the USA (ARTCC managed by the FAA), UK (managed by NATS) and Canada (managed by Nav Canada).
Company frequencies are also covered in this video, along with examples of conversations received at my location in the last few days.