All the above absolutely correct ref CPDLC. London and Scottish (and indeed most of Europe) now use CPDLC in the upper areas (above FL195) and aircraft equipped with the system are encouraged to log on. ATC can sent all sorts of instructions (e.g. climb, descent, heading, speed, direct-to) as well as frequency changes. Think of it like a smart form of text messaging. The protocol for frequency changes is to ACCEPT the CPDLC frequency chance via a flight deck selection and then voice call in on the new frequency. Hence you are only hearing the initial call. The main advantages are keeping radio traffic down on busy frequencies and eliminating transposition errors/mis-hearing of frequencies or other instructions. I recently flew from London to Seattle and was CPDLC from about FL200 in the climb out of LHR until entering the USA in descent from Vancouver FIR.
One of CPDLCs great benefits is it can create a pseudo-radar environment for air traffic controllers for FIRs covering very sparsely populated or vast areas where fitting of radar heads is impossible or impractical. Hence it is used over the Atlantic, Canada and large areas of Africa.
In the North Atlantic (NAT) region the use of ADS-C and CPDLC are mandatory for operations between FL350 and FL390 so most long haul aircraft are now equipped with it. The system makes automatic position reports amongst other things eliminating the need for difficult HF communications. Not having it means FL350-FL390 is not available so most longhaul aircraft have it fitted these days. CPDLC will be soon be mandated in Eurocontrol upper airspace so shorthaul aircraft are getting it now as well.