Overcharging for work not done

My alarm system became dysfunctional in August 2017; on 11/8/2017 a technician checked it and said the siren needs replacing. I was charged R530.00 which I paid. On 17/8/2017 another technician came who replaced the siren and the battery, and I paid the invoice of R1140.00. On 22/9/2017 he came out again for my beam that stopped working. He spent only +-30 min there, said it was the alignment that was out, and he fixed it. Only that evening I realised he left an invoice of R6238.08! On this invoice I'm again charged for the siren and battery (which I already paid for), and now suddenly there is a panel that was supposedly replaced: we weren't informed about this on either of the 2 days he was there, no damaged panel was shown to us, or we're we informed in advance about it needing replacement. No quotation was given in advance, and it was added only almost a month later on the 2nd invoice. I contacted the technician and ADT numerous times and spoke to many different managers (and submitted a written complaint on the ADT website); I was told at the end that the invoice doesn't show on ADT's system and was probably a mistake. Now I'm informed that I still owe them R5098.08. I phoned again, but they can't give me an explanation of this very high bill, for work that wasn't done.