Account reached credit limit and calls suspended

I have been a diligent and reliable payer of mtb for 10 years. I received an sms last night that my credit limit has been reached and my outgoing calls are suspended. I have in the last 10 years never missed a payment. Also no warning phone call to clear up the issue was given before cutting my line. I run an adventure company and had an event today and was unable to phone anyone for logistic or medical emergencies. I have tried phoning your call centre on a number of occasions in the last 24 hours to sort this problem out, the longest call being 30 minutes with no answer. It is appaling that mtn has no regard for their customers. Please can this issue be resolved as I have run out of options.