tbh I think a more realistic depiction would be:

Before UTC4ALL: is UB awake? what time zone is UB in? idk, what zone is Melbourne? +11? uh... carry the one... 6:25, maybe a bit early, let's try in an hour[1].

After: is UB awake? he said gets up at 13:30, so call in a couple hours.

You want to call someone, but you don't know when they're available? Maybe you should ask them, so they can tell you it's 13:30 to 4:00, with zero "is that my time or your time" worries. Or check your shared location-aware calendar, which already handles both cases equally well.

How often do you do several-thousand-mile phone calls without knowing anything about the recipient's schedule? Where I come from that's gonna be rude, send an async message instead.

1: yes, the math/calculated time is wrong. on purpose. as an example.