For a number of years I had a team in five offices in four timezones, so asynchronous working became the norm. We used Skype however. The only "rule" that needed to be enforced was "Away means Away, Available means Available" A lot of realtime problems got stymied by people thinking others were ignoring them.
The "away" rule seems counter productive. The value of async is that I can ask you a question when convenient for me, and you can answer when convenient for you. If I have to wait for your status to become available, I'm wasting my attention.
Basically it means we should trust the "away" message, but still can drop a line if you want. Away could also mean "mentally busy" or "on the phone"
If I could trust the away messages, as a team lead I always had a status board as to where my team was (since we were in five offices)