Re: ICAO codes for Origin and Destination when passing details
RR - I avoided a potential airmiss with a military helicopter once only because he reported (to London Information) his departure and destination "in the clear" and I was able to figure his probable track. We ended up talking to each other, as London has (had?) no radar - using altitude as our seperation. The helicopter crossed 500 feet directly below but never saw me, I saw him below.
Had he been speaking in ICAO, I wouldn't have spotted the potential risk. Remember, R/T calls are (effectively) for everybody on frequency - not just ATC.