Unauthorized direct debit amount of R2770

So in July my mother went and got an early upgrade at an MTN store, she was told the was an outstanding amount of R2.4k that would be spread across the remaining payment term and added onto the direct debits of the new contract. So imagine our surprise as MTN attempts a debit of R2.7k on the 1st Sep, since then we have tried going in store and the consultants cannot explain what has happened we where told to call the call center which is hopeless to say the least, we called a total 6 times 3 of which the consultants hung up without speaking and the other three occasions had the call cut by Patricia and twice by Kabelo Sedumo