Upgraded and line been suspended

I have been on the line with MTN for over an hour being passed from one department to the next. I upgraded yesterday and now my lines have been blocked due to them not increasing my credit limit. I have the phone but cannot do anything with it. I tried loading a top up bundle and cannot even do that. I am very dissatisfied at the amount of time one needs to spend on a call to get nowhere. I need my limit increased so that I am able to continue with normal calls and data! and would appreciate that a person can use the USSD codes that are supposed to be available.<br>