MTN USSD FUNCTIONALITY

Currently I have an contract with MTN with inclusive Data, Voice and SMS bundles. This week I tried to activate a new Openview decoder with USSD functionality witch will be charge at stander SMS tariffs. I was not aloud to complete the process, due to no funds available. I was advice by MTN to buy additional airtime. I then used my daughters phone with Airtime what also did not work. I am already paying for SMS, and to me it is unacceptable to expected to buy adition Airtime to be able to make use of the USSD codes?