DECF = Data Early Cancellation Fee
Example: phoneX retail price ( excl taxes ) is 100$. You see that if you just take a voice with contract that the phone price now become 75$ ( 25$ subsidy ). You also see that if you take a data contract the phone price become 25$ (additonal 50$ subsidy). Let's assume 6 mths down the road you decide you do not want the data option and you request to remove it. You wil then be charged DECF on a prorated basis for the remainder of time left on your data contract.