Instant Messaging is key to modern collaboration; an essential tool for getting the right answers fast. Contrary to a phone call you can see the recipients mode and mood up front and select whether to engage or postpone accordingly.
However, do it smarter than merely setting personal status as "busy", "on phone" or "away". The status helps -- but you can do more:
Open you Personal Settings and modify your "Display Name" to describe your situation. Commuting from California to Maryland and back, I frequently update my description along the lines of "Ulrik - CA, Writing article" or "Ulrik - MD, Meeting with Dev Team".
It is just 2 clicks and less than a line of text. It truly adds to the instant information and thus to the ability of your on-line community to behave politely.