Your contract's already over after the agreed upon time (which is generally a year or two years.) After that yes, you're automatically in a month-to-month agreement, and your explicit consent to the terms of that agreement is indicated by continuing to pay your bill. Those terms may be different from what you originally agreed to. I'll refer you to the terms listed in their Service Agreement:
Term of Service. Your Agreement begins on the day we activate your Service(s) and continues through the Term of Service, typically a 12 month or 24 month period (“Service Commitment”), specified on your Customer Service Summary. At the end of your service commitment, this Agreement will automatically continue on a month-to-month basis. If your Agreement has no Service Commitment, it is a month-to-month Agreement.
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After your Service Commitment ends and you are on a month-to-month Agreement, AT&T may terminate your Agreement at any time with 30 days notice.
1.3 Can AT&T Change My Terms And Rates?
We may change any terms, conditions, rates, fees, expenses, or charges regarding your Services at any time. We will provide you with notice of material changes (other than changes to governmental fees, proportional charges for governmental mandates, roaming rates or administrative charges) either in your monthly bill or separately. You understand and agree that State and Federal Universal Service Fees and other governmentally imposed fees, whether or not assessed directly upon you, may be increased based upon the government's or our calculations.
IF WE INCREASE THE PRICE OF ANY OF THE SERVICES TO WHICH YOU SUBSCRIBE, BEYOND THE LIMITS SET FORTH IN YOUR CUSTOMER SERVICE SUMMARY, OR IF WE MATERIALLY DECREASE THE GEOGRAPHICAL AREA IN WHICH YOUR AIRTIME RATE APPLIES (OTHER THAN A TEMPORARY DECREASE FOR REPAIRS OR MAINTENANCE), WE’LL DISCLOSE THE CHANGE AT LEAST ONE BILLING CYCLE IN ADVANCE (EITHER THROUGH A NOTICE WITH YOUR BILL, A TEXT MESSAGE TO YOUR DEVICE, OR OTHERWISE), AND YOU MAY TERMINATE THIS AGREEMENT WITHOUT PAYING AN EARLY TERMINATION FEE OR RETURNING OR PAYING FOR ANY PROMOTIONAL ITEMS, PROVIDED YOUR NOTICE OF TERMINATION IS DELIVERED TO US WITHIN THIRTY (30) DAYS AFTER THE FIRST BILL REFLECTING THE CHANGE.
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Unlimited Data Customers. If you are a grandfathered AT&T unlimited plan data service customer, you agree that “unlimited” means you pay a fixed monthly charge for wireless data service regardless of how much data you use. You further agree that “unlimited” does not mean that you can use AT&T’s wireless data service in any way that you choose or for any prohibited activities, and that if you use your unlimited data plan in any manner that is prohibited, AT&T can limit, restrict, suspend or terminate your data service or switch you to a tiered data plan.
I'm not advocating in favor of AT&T. I agree with gurps_npc. Hopefully, the FCC will snag them for their unorthodox use of the term "unlimited," which is not how a normal person would interpret that word. They should be required to call it something else other than unlimited. But let's not pretend the contract is different from what it is. Basically the contract amounts to, "we the corporation can do whatever we like, whenever we like, and if you don't like it, you may leave."