If you use online payments or automatic payments, all successful payments are immediately shown in your Payment History, accessible from the Bill & Payments section. Scheduled online payments will show as pending until credited.
Allow up to two business days for your online payment to change the Account Balance on the AT&T online account management Account Overview and Bill & Payment pages. Note: The Bill Amount field does not change until next month when your bill is available.
The Payment History page also displays other types of payments (e.g. mailed in, through your bank, at a payment center, etc.) and gives you the ability to delete a pending payment.