Worst customer service ever !!

Worse customer service I have ever come across!!! If I could give no stars I would! I canceled my contract with them after having to call 20 times to give my device moved to a new car. They finally gave me an amount to pay to cancel, I paid it in full! They are now still threatening me for the a monthly installment that was not cancelled on their systems! Every person that calls me tells me they see my payment and are not sure what the amount is for that they harassing me to pay!! If their own employees can’t explain it why should I pay it ???? I am at my wits end with this company! They are a bunch of useless and unhelpful idiots!