I've found phone calls disrupt my flow, but instant messaging disrupts it a lot more. People are less likely to make a call than to send an IM. As a result, I would get a lot more IM's than calls.

In my last job I had a rule: People who are with me on the site have to come to me physically (or email if not urgent). Remote people can use Skype, but they must call me - not IM. Of course, they can still email if it's not urgent.

People still called, but the volume went down quite a bit.