After checking with Jason, Meredith and lastly with Vince, all with AT&T; local store, National Consumer Support and National Business Support, respectively, my understanding of how to determine the amount of data used, data remaining, billing cycle, etc. is as follows.
Individual consumers; i.e., individual AT&T accounts, can set up a
Management Account online for free where you'll see details concerning your data plan including usage. Have the associated SIM card's 16-digit number and associated telephone number ready when you begin.
Businesses; i.e., corporate AT&T accounts, can set up a
Management Account online for free where details concerning the data plan including usage can be determined.
In both instances, the owner of the AT&T account is the only entity allowed to setup the online Management Account. If your data plan is covered under a business account, you can still find out about your data plan's details, including usage, but you'll have to call the AT&T National Business Support (1.888.444.4410) and speak with a support person. You'll need to have the associated SIM card's 16-digit number and associated telephone number for that specific device ready when you begin the call.