Contract cancellation due to poor service

I have started a 2 year security contract with ADT Security Company during the month of FEB 2017. I have been paying my account regularly and decided during December 2017 to cancel my contract with the company due to bad service from day one. Firstly there were installation hiccups whereby I asked them to install 2 LEDs for me at my business premises which was not done up to date. An FSK radio was installed instead of the normal RF Radio but this was not working properly from day one. The alarm would activate during early hours of the morning and then it was false alarms all along. On top of all of this after reporting the problem about the false alarms, I got billed because the for extra call outs of the armed response. Two months before cancelling my contract I was forced to NOT arming the alarm system due to extra billing. I recently received a bill of R3226,67 which I need to pay by the 31 Jan 18. I personally feel that this is unfair because I have been paying them and I was not provided with the service and feel that ADT was in breech of contract because they did not fulfill their part of the contract from day one. Do I go the legal way and spend more money or what do u suggest Peter?