worst service ever

I have a mtn mychoice100 contract with you yesterday when I received my top up I didn't get my normal 300mb with it after holding on the line for 10minutes without speaking to a consultant I hung up and a few hours later I received my data then this morning I had a emergency and wanted to make a call it said my account is suspended then I received a message saying I have exceeded my credit limit of R800 therefore my account is suspended I don't have a credit limit of 800 I changed that on the 16/11/2014 to R0 when I did my upgrade.so why the hell did I get that message and why is my account suspended i want this sorted out today or else I am going to cancel my contract and go to a other service provider. This is unacceptable.I've been with mtn for 5 years and I won't tolerate this kind of service