I think it's a continuum, for one thing.
Phone calls are clearly sync. But IM on the desktop is, in my working life, much closer to sync than email.
I think of phone texts as attempts at unobtrusive but still mostly synchronous communication.
On the other hand, the group chat for a broader team I'm in is more likely to be used for idle non-work-related chit chat.
And phone calls basically aren't used at all outside of scheduled meetings.
Email and text as async communication only works if both parties actually have the processes in place to reply async.
If people who send a chat message are all asking me ‘if I have time to talk’, life is over if I say yes; it is always no aka async. And for most people I work with and definitely friends, that works well, just strangers usually have to be educated a bit and some never learn (and are just ignored generally).
today i am happly using deltachat, which effectively proves my point.
the primary difference between email and chat is threading. and even that can be handled.