4.7-inch iPhone 6 and the 5.5-inch iPhone 6 Plus
Checking AT&T iPhone Data Usage History
AT&T iPhone customers can view their data usage for the current billing period on the summary page for their account by logging in to the AT&T Wireless website. Such data is only an estimate, however, and AT&T notes that delays of up to five business days may be experienced before data activity shows up in the displayed total. Users can also dial *DATA# and receive a text message with their current billing period's usage.
From their AT&T Wireless account summary page, users can also click on the "View Past Data Usage" link to see a six-month history of their data usage to help gauge which new data plan they may want to select.
For existing customers who wish to keep their $30/month unlimited data plan, they can continue on the plan even if they upgrade to new hardware. If a customer elects to switch to one of the new plans, however, they will be unable to switch back to the unlimited plan at any point in the future. Customers on the grandfathered unlimited plan will also be unable to use AT&T's tethering feature, which comes as a $20/month add-on to the new $25/month 2 GB data plan.