Context: (BrE) - a friend is complaining...
a- "I phoned you three times this morning but you never answered."
b- "I was in the bath, wasn't I?
Why the question-tag, if the listener had no idea what the other person was doing?
I've searched and found that Brown and Levinson regard this use of question-tag as being characteristic of positive politeness. Then again, John Algeo says it's impolite and expresses great annoyance.
What state of mind does it convey? Is there a word or phrase for this form of rhetoric?