Your very first bill is often higher. This is because of the installation fees, which can vary from one request to another.
Also, the monthly fee is payable a month in advance. In addition to the monthly fees for the following month, your first bill includes a prorated fee for the number of days between the date on which you subscribed and the date of your first bill.
You can be assured that your bill total will stabilize as of the second month, as you will only be billed for your next monthly fees and any pay-per-use services, like long distance calls.