If your customers are businesses, they will probably contact you either by phone or email, not by text messages. If your customers are not businesses, its going to be phone and text messages, not emails. So that says what your phone should cope with, and if its the former, you probably want a phone that picks up your emails about as soon as they are sent. That's why BlackBerrys were such a must-have for business execs when they first came out, though of course other phones can also do that now. I doubt if you can find a phone that picks up emails that doesn't also give you internet access because the technology required is so similar, but they might exist.

As far as specific models are concerned: I don't like the tap-once-for-yes-twice-for-no ouija board systems most phones use for text messages, so I have gone with phone with a full A-Z keyboard. My wife, on the other hand, finds that too big and prefers a much smaller phone. Personal taste, you see.