The following idea will make calling much less frustrating and a lot more efficient.
It is revolutionary in North America and other countries - but I have seen it implemented in at least one country (the first part of the idea -- the second part I have not seen it yet):
When you call a party who is on another call, the ringtone you hear is different so you are aware that the party is already engaged in a call and you stop the call and will try later. The party can see you calling but does not have to stop his/her call, or to even write you a text message, as he/she are aware that you got a different ring (example -- broken ringone) and are aware.
How to implement: not sure about the technical difficulty, but for sure it is feasible and already implemented in places like Israel for many years.
Part 2: The same feature would be useful for when you set your phone on DND - do not disturb.
And finally -- going further -- if you could set the DND until a certain hour -- then it would be great if the system can notify the caller - "the party is not available until hh:mm).