If you are signing a contract authored exclusively by one party you can assume it is designed solely to expand and protect the rights of that one party to the maximum extent permitted by applicable law. This can include their rights to modify the terms (usually with some form of notice).

Anything less would be a failing on the part of their attorneys. As a consumer you can agree to their terms or.. take your business to a competitor who will offer similar terms.

If you want some specifics, here they are (emphasis mine):

CAN T-MOBILE CHANGE, SUSPEND OR TERMINATE MY SERVICES OR THIS AGREEMENT? Yes. Except as described below for Rate Plans with the price-lock guarantee (including the “Un-Contract Promise”), we may change, limit, suspend or terminate your Service or this Agreement at any time, including if you engage in any of the prohibited uses described in these T&Cs, no longer reside in a T-Mobile-owned network coverage area, or engage in harassing, threatening, abusive or offensive behavior. If your Service, Product, or account is limited, suspended, or terminated and then reinstated, you may be charged a reconnection fee. Your account may still accrue charges even if the Service is suspended. You are responsible for any charges that are incurred while your Service or account is suspended.

Under certain limited circumstances, we may also block your Device from working on our network. If the change to your Service, Product, or Rate Plan will have a material adverse effect on you, we will provide 14 days’ notice of the change. You’ll agree to any change by using your Service or Product after the effective date of the change. We may exclude certain types of calls, messages or sessions (e.g. conference and chat lines, broadcast, international, 900 or 976 calls, etc.), in our sole discretion, without further notice...

If you are on a price-lock guaranteed Rate Plan, we will not increase your monthly recurring Service charge (“Recurring Charge”) for the period that applies to your Rate Plan, or if no specific period applies, for as long as you continuously remain a customer in good standing on a qualifying Rate Plan. If you switch plans, the price-lock guarantee for your new Rate Plan will apply (if there is one). The price-lock guarantee is limited to your Recurring Charge and does not include, for example, add-on features, taxes, surcharges, fees, or charges for extra Features or Devices.

Probably sneaked in during renewals.