Sorry, I apologize, I misunderstood what you were looking for.
There are a few ways to approach the issue. One is to have a modem that stores usage data that you can retrieve. I am not sure, at this time, if the modems AT&T uses can do that. The next method is to have a router that has software designed to store the usage data for the WAN and maybe the LAN ports. I have a Cradlepoint MBR95 set up for monitoring the Novatel USB modem. I think some of the most recent routers from Netgear, Linksys, D-Link, and Belkin have that function for the WAN port. The third method would be a software package that can be added to a computer to monitor the WAN and maybe the LAN port on the router.
A piece of software for monitoring bandwidth usage:
A wireless router with internet bandwidth usage monitoring:
See page 103 of the manual.I have heard that for some routers you can flash the firmware with DD-WRT »www.dd-wrt.com/site/index
or Tomato »www.polarcloud.com/tomato
firmware and get the bandwidth monitoring.