DEBIT ORDER OVER COLLECTED AGAIN

I took a 2 year contract with Mix Telematics for a Beame. On the 1st August the debit order went through at the old, higher amount. I tried to call but it’s impossible to get through. So I e-mailed on 6th August. I received No reply/ response at all. On 1st September the debit order was incorrect again. I tried to e-mail on 2 September. No Response yet again. This morning in absolute desperation I tried to call. No Reply again. I left a message for a call back. No response at all. What do I have to do to get Mix Telematics to refund the money that they have incorrectly taken out of my bank account and Coreect the debit order. this is disgusting service. it seems that now that you have me locked in for another two years you don’t care to foil your Error. please RESOND