Charging intrest on payments

I have been paying my account with HTN from 2013, after many requested I noticed that the only times they respond to a request for a statement is if I hold back on their payment, now they have been charging me interest for every payment I have made and I have been asking since Oct 2015 for this to be reversed after 3 employees confirmed that these charges are incorrect, I have also been charged with commissioners fees which I will NOT pay!! I am NOT paying for people NOT doing their jobs!!! I am also been charged interest on non-payment for every month that I haven't paid while I wait for them to respond to my query.. this I will NOT pay either!!! I have been waiting for them for 4 months to fix up my account, after 3 months I finally managed to speak to a team leader but still I get phone calls every week and sometimes daily from some uninformed rude call centre agent telling me that I'm going to be handed over or blacklisted if I don't pay... well then be my guest please coz I will NOT pay until my account is sorted out!!!!! they are *********** and terrible!!! I AM NOT PAYING FOR INCOMPETENCY!!!!