What really gets these obnoxious people going is when they call you to ask where you are.
I used to say, "I'm here", and then listened gleefully as they coughed and spluttered into their coffee.
On a real awful note I once upset a work colleague who was very hard of hearing.
He wore a deaf aid in both ears, but during working hours he used to turn the amplifier down to block out the workshop noise when the lathes and milling machines were going full blast.
As he walked to his workbench I sidled up to him and mouthed the words: "Hello, how are you" while making sure not a sound crossed my lips.
With that, he turned his deaf aid full on so I shouted as loud as I could: "Hello, how are you?", which damn near blew his ears off.
Even my mates said how wicked I was and nobody spoke to me for the rest of the day as a punishment.