Poorly trained call center agents who are unable to explain complexities of loan statement.

Have repeatedly asked SA Homeloans to provide me with exact calculation on my installment is calculated. My last attempt I phoned in and went through my statement with a call center agent. I am on the interest only plan . The debit order amount does not equal interest amount + insurance + fees. There is a difference of +/- R1000 which the call center agent was not able to explain. He promised to "look into it" and get back to me when he had answer. No follow up from anyone. This is not my first attempt to get this explained.