[off the bus] or [out of the bus]

"Get on a bus" is the opposite of "get off a bus."

Suppose that your friend is talking to you over the phone while you are riding a bus. He asks, "What are you going?" You reply,

(1) "I'm on the bus."

(2) If you are no longer on the bus, is the opposite "I'm off the bus or I'm out of the bus."?

Most of my non-native English speaking friends think "to be off the bus" is correct because it is related to "to get off the bus." But, my other friends think "out of the bus" sounds better because you are outside.

I am not sure which phrase is correct. Please help me. Thanks.