Overcharging for work not done

This is my 2nd review, still no response from ADT. Extra costs (for parts not replaced) were added to my invoice more than a month after work was done, and ADT says I still owe them more than R5000. Mandla evaluated my faulty system on 11/8/2017, said it was only the siren that needed replacement. I paid his invoice of R530.10 for evaluating the system. On 17 August 2017 Sammy came, said both the battery and siren must be replaced. Nothing else was mentioned or done, and I paid his invoice of R1140.00. On 22 Sept 2017 I phoned him directly when I realised my beam wasn't working: he came out and fixed the alignment that was out; even on his invoice he admitted to only being there for 23 minutes. When I came home that evening I realised he left me with an invoice of R6238.08! On it he again charged me for the battery, the siren, and now a DSC 1864 panel that was allegedly replaced (?on the previous visit?). This was never discussed with me before or in advance, and according to a family member who was present at both visits, this wasn't done. Since the system only costed R8000 a few years back, I would never have agreed to this, and it's very suspicious that it was only added on a month later to the original invoice. I phoned him numerously to enquire what's going on: he promised to phone me back but never did and then blocked my calls (also very suspicious). I phoned ADT numerous times as well; initially they assured me it must be a mistake (also since those panels cost R1400, not R5000), but since December they keep on phoning me and threaten to discontinue their service, saying I still owe them the balance of more than R5000. In spite of again making 3 very long phone calls this week, the technical manager can never be reached and never phones back. I'm trying in any case now to terminate my contract after such pathetic service, but they say I can't do that until my bill is paid. Deeply regret ever using them.