Because they are liars
They usually claim that you are billed 1 month after you use the service so they will always charge for that month.
Probably because you canceled your service in the middle of the billing cycle
because optimum charger you one month ahead so if you want to cancelled the service you need to cancel the service one month before the due day
Is the bill ciclo thats what you ow.
Because they want to make you more miserable for leaving them. When they sell you the service they tell you one thing. When you cancel, they transfer you to 6 different people that give you attitude over the phone. Eventually you are told that there were changes made "last month" and now you must pay for service you're not using because you should've told them sooner. It's all B.S. just like their services. Wait till they start to send you promotions every month now that you've left. Go to FiOS, you'll pay a little more but you won't deal with their garbage.
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