Writing "COME NOW!" takes two-three seconds, calling is nowhere near that.
Video (in French but pretty clear visually): https://www.youtube.com/watch?v=LMaiUS6jkn0
I agree this could be useful in some cases.
I argue that calls shouldn't have to be synchronously initiated, e.g. I should be able to join a voice channel once I'm available and the other person should be able to join whenever they're available. This saves a lot of pointless back and forth (can you believe some platforms still don't directly connect people that are calling each other, instead falling both attempts?!).
An easy response to that would be "but I want to be able to send the other person a strong message they have to act on so that we can synchronize". This is an important use case, but you see, it doesn't just apply to voice calls! Why shouldn't I be able to demand someone's immediate attention to a text message?
If I was implementing this feature, I'd probably use the phone's voice call API for the notification.
It would be great if the app detected that (for instance) something is said in ALL CAPS (or with an exclamation point, or perhaps some more indirect, non-text-dependent, flag set but that sounds annoying) and _changed the notification sound_.
It would be great to signal that I need to e.g. stop cooking and look at the phone now, because something is happening elsewhere in the house, for instance.