You may notice that your first AT&T bill is higher than your normal monthly rate.
Bill charges
Monthly plan rates are billed one full month in advance. In addition to your monthly plan rate for the first full month of service, there may be a partial monthly charge covering the period between the day your service was installed, and the start of your regular monthly billing period. Also, your bill will include taxes, fees, and surcharges, and all or part of your installation charges, if any.