Half and half? :)
I'm not sure if you're talking about a sales or customer support perspective, but from a sales perspective:
I think most people are getting overwhelmed by email. Lots of folks get 100+ a day and I know I'm not the only one who can't necessarily respond to all email within a reasonable amount of time (say 48 hours).
My phone is even worse though, and I've got three numbers... my office which always goes to my voicemail (which goes to email), my "direct" line which is given out sparingly (but generally answered if I'm at my desk), and my cell phone which is given out even more sparingly, always left on silent, and very rarely answered unless you're family or personal friends or maybe the boss.
For me, at least, a cold contact via email is much more convenient than by phone (chances are I'd never return your phone call). You probably have a 2/3 chance of getting an email response from me, even if it's a thanks but no thanks.
For customer support, I expect to be contacted in the mode I contacted you in, unless I specify otherwise. Generally I prefer email, but sometimes the phone is necessarily just for clarity's sake.
I no longer think of e-mail as an effective communication tool.
It's still effective (and my method of choice), but the way I manage it has changed dramatically over the last five years.