@chevvron & Not Long Now:
The point I'm trying to make is that sending a selcal tone is not rocket science.
What ATCO's don't seem to realise is that monitoring 121,5 has become very impractical.
Very often when a pilot asks you to "say again" your instruction, the reason might well be because something or someone was transmitting on 121,5 at the same time. It's like listening to two songs at the same time! 121,5 is scrambled with pilots calling dispatch, false ELT warnings, ATC trying to reach lost com traffic several times, people telling jokes or playing music on it and static interference.
My point is that selcal works nicely on VHF and it should become mandatory for airliners to maintain selcal watch on 121,5 iso of having to monitor it which causes more problems then it solves.
I don't want to hear lame excuses like "It can't be approved unless is costs several hundreds of thousands of whatever currency" or "I'm not allowed to take my cellphone in the ops room"
The fact is: The technology is very simple and there's no reason why it should be expensive to implement! Instead of being negative towards this solution, ATCO's should be supporting this. The result would be "less flights with lost com and less scrambled fighters and improved safety". Who can be opposed to that?