MTN - not able to make calls

Hi, I have been an MTN client from 1999 with the same number. I am current not able to make calls, as it says insufficient funds. I am on a contract have do not have any set limits. I have also not authorised any limit on my phone usage! My phone bills have varied from R400 to R4000 in the past. As a business person, this service is really not acceptable, to be cut off without a phone and no one from the 808 number able to help. I have called this 808 about 10 times today, from 7 am this morning. I am currently holding on. This is the worst service I have every had. I wish I am able to email MTN CEO with this regards to how MTN does not want to retain their clients. I am due for an upgrade and really considering moving to another service provider. Can someone PLEASE help?