No Airtime

I currently have a top up 500 account with MTN and each month my account closes the 7th of each month and the 8th they send me an sms telling me that my account has been credited with yadda yadda amounts. On the 9th August I tried to make a call and was told I have insufficient funds to make this call. I tried calling the call center but got the usual \we are experiencing high call volumes"message. I tried the 10th of August the Sunday and got through. The consultant reassured me that the matter will be investigated within 7 days I need to call again. I have been calling ever since and yet to no avail has this matter been sorted. After numerous attempts to resolve the issue I feel I want to cancel this contract that I have had with MTN for more than 10 years and move to a different provider. WHAT A JOKE YOU ARE MTN !!!!!??"""