Charged for SMS but already paid

Hi, Impact Meter always sends SMSs every month saying I am in arrears. I have never missed a payment and I always send them them proof of payment, yet, they still send me an SMS saying I am in arrears. They then charge me for these SMS's. When I called they say it's the head office that sends and there's nothing they can do about it. What kind of service is that? Will I be charged for these SMS's until I move? Please make it stop. Regards, Zez1000