I've written about this before, but had two recent instances where someone needed something from me on short notice (i.e. within a couple of hours) and emailed me instead of picking up the phone. In both cases, they were confused and disoriented when I explained that I had been in meetings and do not check my email in meetings.
If they had called and left a voice mail (or spoken with our receptionist), I would have been able to get them what they needed.
I get so much spam that I rarely check email on my phone--deleting spam on an iPhone is painful. And yes, we have spam filters running on our mail server, but about 3-5% gets through, and that's several hundred spam emails a day.