Telkom - having to pay for line usage even if they cancelled it in advance

17 April 2023 - I cancelled my mom's landline as she does not make any calls anymore - she is 81 years old. As per the webpage, I gave a month's notice so expected the phone to be dead from the 17th May 2023. I informed the family and friends of this date. My sister tried to phone her but the line has been cancelled already. When I phoned Telkom, I was told it was cancelled on the 18th April already. I asked about the line that you pay in advance and was told on the final invoice it will show the discount. I got the final invoice today and only a penalty is added because of late payment and no discount for a line that was charged, but could not be used. Today, the Telkom agent told me that because the invoice went out before the line was cancelled, that mom is liable to pay for that. This means she is paying for something she could not use. This is of no fault of us - I gave a month's notice period. Telkom need to adjust the invoice so that she is only liable for the rent of the line from (as per invoice) 12 April 23 - 18 April 2023 and NOT to 11 May 23