***** by Mr Price Store regarding contract cellphone

I have a problem, i bought a cellphone on a contract at Mr Price Store. So the problem started when they didnt do a debit order on the first go, i ignored it at first but months went by and they were still not doing their debit order so i decided to go to them and asked why they haven't been doing the debit order they just told me that maybe it my bank but i told them that they are not the only people making the debit orders in my account and i asked them why they didnt send me an email regarding that and they said nothing. So now they are sending me messages telling me to pay a very big some of the amount because i haven't been paying which is not my fault what so ever because it them. The second time i went back to the store to talk to the manager but the lady that was helping me made a call to the manager and they took my IE number and a photo of my cellphone contract and the manager said i should not pay anything they will fix this matter, but still that didnt do any change still they send me messages demanding for me to pay. I told them that they are messing up my name with what they are doing because it not my fault and they giving me final warning for a problem i didn't course. My thing is that if i pay this money this will just show that i know about me being in arrears which i dont want to do it because im not at fault here.