i need to pay for a system fault

I did a upgrade 3 december, 2 phone, ( my mother and my 1 son) they did not have the phone in store so i had to go back on the 10th to finish the upgrade, when i got my bill on the 15th the upgrades were wrong, they upgraded the wrong numbers, they upgraded my two sons and not my mother and son, ( my other son should not had upgraded because he wasn`t due) they had system problem on the 10th because of the loadshedding)

now when i got my bill on the 15th of december my bill went from a normal bill of R1530 to R2480, so i was billed R952,18 extra on my bill for a system fault

from then me and the consultant that help with the upgrade tried to sort this out with mtn but could not resolve this

i went to mtn today to go and sort this out, the consultant or manager not sure who she was would not understand that the fault is on their side and not my and just said that it is pro rata money that i owed to mtn and should be paid that is the reason why this could not be resolved, the system put the pro rata money that is usually paid over 3 months on my bill as one amount,

im just not sure how i can be charged R952.18 pro rata for 1 day on 2 phone

when i look at my bill i see that in december i was charged from 11-12-2022 to 11-01-2023 and again on my january bill im charged its now 12-12-2022 to 11-02-2023

and also the rest of my contract was billed almost double in december without increase

and all i get from mtn is that they do not give credit and i must pay the money