Customer Service cant resolve issue

My phone will neither receive or make calls. When trying to make a call it says I have insufficient airtime so I loaded airtime. Still get the same message and when I call the phone it says the number is not available. I can however recive the irritating recorded telesales calls from MTN. So I phone customer services and am told they can see I loaded airtime and say that they will reset the system......still doesn't work I wait the prescribed time frame.....still doesn't work Phone customer service again and another set of promises......still doesn't work Call customer service ....same promises ....still doesn't work Call number 5 .....this time I am told they will speak to a manager and call me back within an house......still doesn't work..........still waiting for the callback 5 calls over 2 days and the issue is still not resolved. This is a number I have been using for business for 3 years so can not be without it but Customer Service can not resolve the problem......... I am frustrating beyond words and although the customer service agents are very pleasant and friendly they are ill-equipped to resolve the problem and off course promising call backs and not doing so is quite unacceptable