Yeah. A lot of it is convention of course. But where I work, Google Chat for individuals and small groups has sort of developed in general into a higher priority interrupt than email. e.g. are you joining this scheduled call?
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.
Its called "Instant Messaging" :-)
So I think the expectation is built into the name, even if you don't agree.
I would have to disagree. To me "Instant Messaging" != "Instant Answering". It's only a guarantee that the message is delivered instantly.
And unfortunately there is an implicit expectation that the reply is to be sent immediately as well.
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.