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When T-Mobile announced price hikes of up to $5 per line on older smartphone plans last month, many customers were shocked because of T-Mobile's years-old promise that their price would never rise as long as they stuck with the same plan.

"New rule: Only YOU should have the power to change what you pay," T-Mobile said in a January 2017 announcement of its "Un-contract" promise for T-Mobile One plans. "Now, T-Mobile One customers keep their price until THEY decide to change it. T-Mobile will never change the price you pay for your T-Mobile One plan."

Unfortunately, the promise wasn't as simple as T-Mobile claimed it to be in that press release. T-Mobile also published an FAQ that answered the question, "What happens if you do raise the price of my T-Mobile One service?" It explained that the only guarantee is T-Mobile will pay your final month's bill if the price goes up and you decide to cancel.