>Does that make sense?
No, it doesn't work that way.
>If the listener expects the speaker to be on a particular bus (e.g.
>it's morning and the listener knows that the speaker always takes a
>bus to work), then "on the bus" is the right choice.
No. The listener doesn't have to know or expect that.
Even if A knows that B drives to work and B knows that A knows that, B's answer should be "I am on the bus." in
A: Where are you?
B: I am on the bus. I cannot talk now.
Some people drive to work, some take THE bus. (Not: Some people drive to work, some take A bus.) I was on the bus because I took the bus. I took the bus means I went by bus.
A bus is a type of four-wheeled vehicle or a bus line.
If it is rather
>unexpected for the listener to find that the speaker's going by bus
>somewhere while the conversation is taking place (this is going to be
>a bit complicated: the speaker knows that the listener thinks that the
>speaker should be at home, watching Słabysz on TV), then "I'm on a
>bus" is the right phrasing for me.
No.
I am on the bus, or for example, I am on a bus heading for...would be the right answer.
A: Where are you?
B: I am on a stinking crowded bus. I cannot talk now.
B describes the four-wheeled vehicle.
A: Where are you?
B: I am on the stinking crowded bus. I should have taken the train, as you told me. Anyway, I cannot talk now.
Before B set off for the trip, he had a conversation with A. A wanted B to take the train, not the bus, but B thought the #31/10:30 bus would be more convenient. A knew the (#31/10:30) bus would be crowded and warned B not to take it, but B wouldn't listen.