I'm on the bus
Said in two different environments:

1. When commuting on the bus and one is engaged in a mobile phone conversation, it is used to avoid talking loudly about embarrassing topics in a crowd of eavesdropping fellow commuters.

2. When person A is tired, or doesn't see the logic of why person B has called, this can be said to avoid conversation with person B.
1. A: "Hey what did the doctor say about that lump on your balls?"
B: "I'm on the bus."
A: "Oh aight."

2. A: "Hello?"
B: "Hey, will you be hungry in half an hour?"
A: "Aren't I meeting you in half an hour?"
B: "Yeah, but I want to know how hungry you will be."
A: "I'm on the bus. See you soon."