Erudite conversation

When you call somebody after a bad day’s work, an exuberant ‘Hey’ on the other end of the line always manages to lift your mood. You know the person has recognized your number, knows that you are calling and is happy to hear your voice.
I called up my roommate S on her mobile from my office a while back. The usual mundane conversation ensued after the happy-to-hear-your-voice ‘hey’.
S (enthusiastically): Hey
Me (half-dead and surrounded by piles of incomplete work): Hey. Where are you?
S: I am in office. Where are you?
Me (confused because I thought she had recognized my office number on her caller id): Do you know who this is?
S: Yes. Obviously.
Me: Who?
S (getting impatient): Obscured
Me (totally baffled): How did you know it was me on the phone?
S (exasperated): You call me from your office everyday you moron. I recognize the number.
Me (still confused owing to the brain-dead state): Then why are you asking me where I am?