In my experience, the open office idea only works well if you've planned your company around it. I worked at one place where we were all open, but by convention, everything short of a true "the site is down" emergency started with asynchronous communication. IM and group chat, mostly. If it took more than a few messages, it might move to verbal, but it always started in a form I could ignore for a while if I felt like it.

Unfortunately, that only works if it's part of the company culture and each new employee sees it pretty clearly.