Worst Customer Experience with Telkom

Hi HP, I need you help here - I have been a very loyal customer of Telkom Landline Service since starting of 2016. In the month of of Sept 2018, I have shifted my residence. I went to Telkom Service Center and raised a transfer request one month before I shift. The Service personal informed me that they will work on it on time so that I can avail uninterrupted service. But in reality nothing happens as such. I kept on following them thru email, calling customer care and visiting their service center till 1-2 months after I shift, but I did not hear back from them. They only kept sending monthly invoices. I decided to cancel the connection and NOT to pay the bills for the service that I have not used. Please note - I have cleared all the payments till I used their service. Now I received calls from the Debt collector services threatening me for reporting to Credit bureau or summoning me. It is highly unfortunate that - I never heard back from Telkom but getting calls from Debt Collectors. I feel it is total unjustified behavior of a Organization to their customer. Please help me in this regard. Many thanks.