cancelled account and but still being billed

I had an account with chubb securities 2 yrs back when I bought my house and used their services..(Acc ********** 402) 3 months down the line the system stopped working due to lighting. I then called chubb informing of this and i was told that i need to pay for something so that they can come come fix it as its not covered with my policy. I was then unhappy with that I then called chubb to let them know that i would like to cancel their services. I was then told I need to send an email for which i did. then one of the chubb guys came over and disconnected the alarm system. i was made to sign a form confirming that the system was removed. i did and handed it over to the man and the system has not been working since then. I was told that money will be deducted on my account. strangely they continued deducting my monthly fee for which i went to the bank to stop the debit order. fast tack to 2017 last month i received and sms from a company called Nudebt saying that i need to pay 499 for this account. which i then immediately called them back as i did know what was this amount for. the guy then said this is what he got from chubb and that is the reason why i received the sms. I then explain this to the guy the whole story and he said this could be that maybe chubb didn't get the cancellation fee amount. i then told the guy to go back to chubb and asked them for my records since he didn't have the full story I gave him my details and he was meant to respond back to me I m still waiting for the email/call confirming this. since i was waiting i then decided to make the payment of 400 with then intentions of making the rest of the payment once we are sure what the R499 was for. strangely this month again i now got another sms from nu debts saying the i now need to pay R1485. I made it my point to call again and find out what was this amount for and the lady that assisted me this time said she just received this new statement from chubb and i asked but what is this amount for now as i have made the first payment she is like this is the billing for the past 2 year, I then explained the story again to her and asked how do I pay for something that was cancelled 2 yrs back and has not been working. she wasn't able to give me an answer i then asked her as well to go back to chubb to find out what is the amount for and still she also has not gotten back to me. on Monday i then receive an email from nudebt saying this "Our client further instructs us that notwithstanding numerous written and telephonic requests for payment, you have failed, refused and/or neglected to comply with our client’s requests for payment." so as a client I want to know which communication is this and let them produce all the telephonic request that they are referring too as have not been getting calls from them and also why have i not been given feedback about my queries from both Nudebt and chubb. I refuse to pay for an amount that i m not even sure what is it for and if this is the money was owing why was this not included with the first sms that I received from them? why were these 2 amounts not been added together. also what i find strange is that why are there 2 account of the same property as i still get the letters from the previous owners of the same account/ house? now i am receiving 48 hr and 24 hr smses to say i need to pay or the matter will be escalated. why can they not provide new debt with my email when i was cancelling their service and the letter i signed when the installation guy came and disconnected the system.