Dear Coworker:

When I'm trying to negotiate with someone on the phone, do you know how distracting it is to hear you muttering over at your desk in complaint about the person on the other end of the phone? You do realise that you bitching about what you imagine their problem is makes it really difficult for me to pay attention to what they're saying and respond accordingly, right? You do know that this is 99% of why I don't like making phone calls when you're in the office with me, don't you?

Sincerely,

hirondelle