As reported by The Consumerist:
T-mobile [raised] overage rates September 1st.
Customers can use it to cancel without early termination fee.
For single line plans under $59.99 and pooling plans under $89.99, overage rates increase to $.45. All other single and pooling plans increase to $.40. The rate increase amounts to what is known as a "materially adverse change of contract," which, under standard contract law, voids the original agreement, making it so customers can leave their cellphone contract without the usual early termination penalty.
... they may say things to you like, "you have to have had overage charges recently for the change to affect you". Or,"that based on your plan history, the change won't effect you." That's not how materially adverse changes work. It's if it could effect you.
If you need it, here is the relevant paragraph from the T-mobile contract to read:
Changes to the Agreement or Charges. EXCEPT TO THE EXTENT PROHIBITED BY LAW, IF WE: (A)INCREASE THE CHARGES INCLUDED IN YOUR MONTHLY RECURRING ACCESS RATE PLAN, OR (B) MODIFY A MATERIAL TERM OF OUR AGREEMENT WITH YOU AND THE MODIFICATION WOULD BE MATERIALLY ADVERSE TO YOU, WE WILL NOTIFY YOU OF THE INCREASE OR MODIFICATION AND YOU CAN CANCEL THAT SERVICE WITHOUT PAYING A CANCELLATION FEE (WHICH IS YOUR ONLY REMEDY) BY FOLLOWING THE CANCELLATION INSTRUCTIONS IN THE NOTICE. IF YOU DO NOT CANCEL YOUR SERVICE BY FOLLOWING THOSE INSTRUCTIONS, OR YOU OTHERWISE ACCEPT THE CHANGE, THEN YOU AGREE TO THE INCREASE OR MODIFICATION, EVEN IF YOU PAID FOR SERVICE IN ADVANCE. IF THE NOTICE DOES NOT SAY HOW LONG YOU HAVE TO CANCEL, THEN IT IS WITHIN 14 DAYS AFTER THE DATE OF THE NOTICE, UNLESS A LONGER PERIOD IS REQUIRED BY LAW. EXCEPT TO THE EXTENT PROHIBITED BY LAW, CHARGES FOR PRODUCTS, SERVICES, OPTIONAL SERVICES, OR ANY OTHER CHARGES THAT ARE NOT INCLUDED IN YOUR MONTHLY RECURRING ACCESS RATE PLAN (SUCH AS DIRECTORY ASSISTANCE, ROAMING, DOWNLOADS, AND THIRD-PARTY CONTENT) ARE SUBJECT TO CHANGE AT ANY TIME WITHOUT NOTICE, AND IF YOU CONTINUE TO USE THOSE SERVICES, OR YOU OTHERWISE AGREE TO THE CHANGES, THEN YOU AGREE TO THE NEW CHARGES. VISIT OUR WEB SITE, RETAIL LOCATIONS, OR CALL CUSTOMER CARE FOR CURRENT CHARGES.