I agree with the overall thread that visual voice mail is great and doesn't really use that much band width. There is, however, another reason to remove it. AT&T has a service call unified voice mail messaging. This allows your home voice mail and your wireless voice mail to be unified. In other words no more worries about missing a message at home, because you never check the home VM. All your voice mails go to the same voice mail box.
Here's the catch, unfortunately the UVERSE voice mail system works by sending your VM to a different voice mail server than the usual wireless voice mails. For reasons I do not understand this server is NOT compatible with the iPhone visual voice mail. In other words unless you de activate the visual voice mail that button will be totally useless and won't even tell you how many voice mail messages you have. There is a work around which involves downloading AT&T's nifty VM Viewer, which in some ways has better functionality but is very buggy (ie crashes and takes forever to load up).
In anycase I think the following procedure inactivates the VM and redirects the number dialed to the one your current voice mail uses.
Please note that you will need to substitute your VM number for the 18583526900. In other words you need to ask your company what your VM dial up number is.
iPhone Instructions
*5005*86*18583526900#
CALL
HOME MENU
SETTINGS
GENERAL
RESET ALL SETTINGS