Phones cut for "short" payment on incorrect billing

Good day. I AM BEYOND LIVID. I have been a loyal customer since 1998!!!!!!!!!!!!!!!!I have 5 contracts with MTN. I pay R2400 per month. I have a letter from MTN Mondo, stating the amount I should have been charged. I was charged incorrectly. MTN did not fix up their billing. So, I short paid by R400. I now made a payment arrangement to get the phones on, but still its not working and I am told I have to pay it NOW!!!!!!!!!!!! Why should I be negatively affected by someone's incompetence. I phoned 5 times yesterday, 3 times I was cut off!!!!!!!!!!!!!!!!!!!!!! NO MAN, I ALWAYS PROMOTE AND SUPPORT MTN BUT THIS IS TOO MUCH!!!!!!!!!!!!!!!! Sort yourself on and get my phones and contracts reconnected ASAP!!!!!!!!!!!!!!!