Terrible Service as a result of incompetence

I need help in understanding just how MTN works because for many years my debit order for invoices loaded on the 10th or 11th of each month is due for processing on 30th or 31st of each month. This month however despite being in a credit my phone was blocked for outgoing calls on 14 Sept 2015, after my invoice was loaded on 11 Sept which meant that the normal debit order amount was lesser than usual. I have sent several e-mails and have had absolutely no response, calling accounts department is a waste, as it does not accept my contract cell number which means that all my efforts to contact MTN has been in vain. I need to know if MTN has changed their terms because clearly invoices are due within 1 working day of an invoice being loaded and not month end as it was previously. From the comments on Hellopeter Mutshinyani is making many customers unhappy.