Billed for a Line i do not have anymore

on the 13th December 2015 my employer installed a Telephone line at my home with ADSL services. The Line was installed by Arthur who is a contractor for Telkom.Since december i do not have access to my old phone line anymore which is *** as only the one line is working now that the employer had installed. For three months i did not get any invoice from telkom for the old line as my expected my old line to be cancelled when the new one was installed. Last month i get a invoice stating that i owe them R842 for my telkom line. I logged on and created a dispute where i haven to heard from them. A few days later i get an email stating i owe them the R842 and i created another dispute which has not been answered. Today i get an invoice again stating i owe them R1061 with no contact from them on my dispute. I want to know how can they bill me for a line i do not have access to anymore as i have been using my new line. The contractor disconnected my old line. I am not paying the invoice until i have and answer from them. They want me to pay but do not come back to me when i submit disputes.