Very bad service from MTN

I have been dealing with a very helpfull consultant from MTN Mbuso Nhlapo from MTN Cosmonet. I upgraded two of my contracts and moved a 3rd one from Vodacom to the MTN Network. The one line was in use as my office data and to use my VOIP telephone system for the last 2 years. After the upgrade my Voip system stopped working. I puit this through to Mbuso who on his side referred it for a resolve. MTN just shrugged their sholders and said sorry it is not available on their network anymore. I dont accept this there is thousands of VOIP systems running through MTN. My own service supplier said he could provide me with an MTN data package to use with their VOIP system. How is it possible for them to provide me with a MTN package but my current package with the same service provider is suddenly blocked of the service and without any notice from MTN. Please explain this logic to me.