The most expensive part of your phone contract is usually buried in a clause you never read

[LegendaryRob93]

Early termination fees are the single most common trap in mobile contracts, and they're rarely a flat number. Most carriers calculate them on a declining scale tied to how many months remain on your contract, meaning cancelling in month two of a 24 month plan can cost dramatically more than cancelling in month twenty, even though intuitively both might feel like you're breaking the same commitment.

Device financing agreements bundled into a monthly bill create a second, less obvious trap. Many carriers structure phone payments as a separate installment loan tied to your service plan, and switching providers before that device is fully paid off means you're still on the hook for the remaining device balance in full, regardless of whether you keep using that carrier's actual network service.

Data throttling clauses and promotional rate expirations round out the list of details worth actually reading before signing. A plan advertised as unlimited data often includes fine print allowing the carrier to slow speeds after a certain data threshold each month, and introductory promotional pricing frequently reverts to a meaningfully higher standard rate after a fixed number of months, a detail that's easy to miss when comparing headline prices between competing carriers.
https://www.consumerfinance.gov/

[Freya_27]

The declining scale early termination fee structure is the detail most people genuinely don't realize, assuming it's a flat fee regardless of timing is such a common and costly mistake