Really bad service..

I have been a MTN client for over 5 years, i have 3 contracts with them, in december 2017, i got paid late so my debit returned, i paid in R500 straight away and settled the balance on the 17th or 18th of dec, my bill comes around R1290 or R1300 every month, i pay in arrears as that is how my account is. My december bill came to R2044 whch i was debited in january, this amount was incorrect, I called in and consultant logged query ref number 1- ********** 9334, i then received an sms saying query resolved, i then reversed the R2044 and eft R1320 as this is the correct amount, i get an sms today stating i am in arrears, so nothing was resolved, i call in today and logged another query, i was told reference number will be smsed to me, nothing has been smsed, such a big company yet such bad service, how can i be incorrectly billed, was my december payment not taken into account, can anyone help me????????