Forced to pay a bill for a non existant landline

I am reporting on behalf of my Mother Margaret, we have had a Telkom land line phone ***) for years now but my mother started experiencing problems with the phone not functioning and next minute it is working again. My mother went to Telkom at Maponya mall and she got a very bad service, she requested to have the phone completely closed and from the 9th November 2014 the phone stopped working and it was confirmed that the phone has been discontinued thereafter she was told that the phone is of her late husband and she can not discontinue the phone he must. Now she has received a letter to pay a bill of R21'000 for a land-line phone she has not been using and no calls have been coming through because the PHONE DOES NOT WORK!! this is unacceptable she will make no payment as such. If the land-line has been accumulating money monthly then someone has definitely bridged that land-line number and has been using it whilst it reflects my mother's name.. we will not pay an amount we do not know i refuse you will investigate and come to the bottom of this. I have never all our years using Telkom we experience such bad service and given no choice but to pay money we have no idea of