From the Customer Agreement:
"If you cancel a line of Service, or if we cancel it for good cause, during its contract term, you'll have to pay an early termination fee. If your contract term results from your purchase of an advanced device, your early termination fee will be $350 minus $10 for each full month of your contract term that you complete. Otherwise, your early termination fee will be $175 minus $5 for each full month of your contract term that you complete."
For example, if you complete 12 months of a 2yr contract for a smartphone, your ETF would be $350 minus $120(12months x $10/month) = $230.