I referred to meetings as synchronous because they acquire a lock on everyone's time, at a specifically scheduled time on their calendar.
I refer to Slack and e-mail communication as asynchronous because you're not explicitly locking other people's calendars, but those other people are expected to read and respond to the notifications all throughout the day.
If you never know when the next important "@here" conversation is going to pop up in one of your Slack channels, the only way to have a seat at the table is to be always on, always connected.
If someone sends you a message, and they’re happy to wait for a reply, then that’s asynchronous.
If someone posts an @here in a channel asking for comments before making a final decision on the spot, then that's synchronous.
Asynchronous communications must be structured fundamentally differently from synchronous ones in order to be effective. Trying to merge them into a single mechanism is a horrible mistake.
I find phonecalls less disruptive, they happen once, and the issue is addressed. If it is dragging too long, we move it to a meeting.
A chat session can continue indeterminate amount if time, keep flaring up and distracting you as different people chime in from half-afk half-paying attention standpoint.
Nit: I've always heard this called synchronous, since in this style work is performed immediately, rather than queued up to wait for execution at a scheduled time.