Incorrect billing

I have taken out a loan with capitec bank over the phone, everything was done over the phone from the beginning, I recently called in to find out the settlement amount for my loan, and then they tell me that my loan that was taken out for 36 months is now 42 months. and they cant tell me why, i am very upset with them as they claim to be the best and they not like other banks , but to me it looks like you treat me just the same. I demand answers of why I am being billed extra amounts rather than the initial agreement.