no voicemail since December

I have had problems with my voicemail since December 2014. When I call 100 to set it up it always says the number is not available. I have called the call centre numerous times. Nobody seems to care or take responsibility. I even went to my local MTN store and had no better service. Twice after about 5 calls to the call centre I could set up my voicemail only for it to stop working within a few days. I am losing a lot of income this way as lots of clients only have my mobile number. I am at my wits end and am very frustrated that nobody at MTN takes responsibility or call back to follow up when they promise they would. What recourse can one take!?