Mtn account

Good day on the 3/06/2015 I ask then to sent me letter with the outstanding amount I sent me a letter with the amount of R6205.43 and they layout the following: Capital R4975.51, Interest R2284.33, Fees R2345.59 , Payments R3400, I did paid then on the 30/06/2015 the amount of R6205.43. but yesterday I receive a call for the call centre and tell me there is still money outstanding for admin and interest fees, how can they add daily interest to an account ! I try to clear my name from the ITC list, I struggle every day I don't have money to waste because I am single mother I lost my husband and son in 10 months time, and I have high school daughter to support . Previously they phone after 19h00 at night and even after 13h00 on weekend, and if your arrange an payment with then, they still phone you every day and every week and we must the phone call and that is not fair to us