Cancelation refund

I canceled my contract with MTN. Because it was an early termination I had to settle my outstanding 16 months. My installment per month was R405. I received a quote to cancel my contract of R6900. If I multiple my R405 x 16 months, it R6480. No one could help me at the time because apparently thats just what the system generated amd nothing anyone from MTN could do about it. I just payed it because at the time I was in a hurry to cancel. According to me I am only supposed to settle my device and not the whole package. For example if my device was R300 out of the R405, R300 x 16 months is R4800. It feels like R2000 was ****** from me. This is unexcepatable.