rejected debit for an amount was not advised of

my line was activated on the 31.11.2020 my first payment was going to be the 01.01.2020, as advised by sales person , my monthly payment will be R650,i left enough money on the account for me first debit , which I got sms from my bank it was not successful, i tried to call customer service no one picked up , i payed manually to their FNB acc, now i get sms i owe a balance of R683 , what is that for , why is the invoice showing R1298, when i was suppose to pay R650, if I was going to pay more first month why was i not advised so I can have enough money , no having rejected debit ,

I'm so unhappy right now , i need someone to call me and explain what is going on