I don't know if everyone has one of these contraptions these days, but many desk jobbers are forced to respond to the beck and whim of the dreaded office phone. It's a voicemail receptacle, an instant communicator for those ignorant of IM or email, and a major distraction against productivity.
I rarely use my desk phone to make calls and I've come to dread it's familiar ring. To help cut back on calls and help increase my productivity, I have permanently enabled the "Send Calls" function on my phone. This function is found on most desk phones and sends all incoming calls to voicemail.
Now, it's not total communication shutoff -- you hear half a ring, the caller's name appears, and you have about 10 seconds to answer before voicemail takes over. On the caller's end, they hear a beep and then a ring; this also occurs when you're on the other line. As such, most people associate this sound with the latter and think you're on another call.
Most people, upon hearing this, either hang up or leave a voicemail. There's a slim group that I always pick up for, but a majority of my calls I don't answer anymore. The majority of those I ignore send me an email, and a slim few leave a voicemail, while one or two choose to pay my office a visit...!
For the most part, this has become an incredible filter for me and has saved immense amounts of time. I like this method better than a rambling voicemail greeting about my preference for colleagues to email me; it's not as rude and allows you a chance to answer the phone. Do you have methods of dealing with your phone calls too? Does your desk phone have a Send Calls button? Do you even have a desk phone?