For a Full BES you need a Corporate BlackBerry dataplan or it will not work.
BES Express 5 needs BIS or BES dataplan to work.
With out it you will bang your head trying to make it work.
What carrier are you asking about?
You can search BlackBerry DataPlan BES to know more as it has been discussed for years now and has not changed.
Sprint has a neat blurb http://www.sprintusers.com/forum/sho...d.php?t=159510