Are you on auto-pay?
There is a lag in the system. If you discontinue some service, or they remove some erroneous monthly billing, the auto-pay will still go at the same amount for at least one month. You will wind-up with a credit balance.
Best fix is to temporarily shut off auto pay. You have to CALL a rep to do so, you CANNOT do it online!
It is like pulling teeth to get them to admit to this. They tried to tell me that a partial-month credit for discontinued TV service would reverse the extra amount being billed on the auto-pay.
No, it was a partial-month credit. They were going to go ahead and bill me on auto-pay for service that I no longer have.
MAYBE after my next billing cycle, they would have credited me for the over-payment on auto-pay for service I no longer have.
My next bill, I will pay by check, once I have verified accuracy, as I have managed to shut-off auto pay, after the usual long wait on the phone.