Quote from galaxy flyer:
"Funny, in the US, eons ago, i was taught to insert the code without reply. ATC can see the code change and, if incorrect, make a radio call. Theres no need to say anything, just put the code on."
Sure that will work fine if their screen was reasonably uncluttered, but otherwise they might not notice that the wrong a/c had picked up the call and changed codes. Also - and I'm way out of date on this - if a number of a/c are coming in from oceanic simultaneously and all squawking (say) 2000, the same error is likely to happen sooner or later. Think it's fair to say that ICAO R/T works on the principle that ATC instructions must be acknowledged, and the latter must include the a/c callsign. If everyone is in practice and on the ball the air time taken is minimal.