Unhappy Vodacom client

Last month a company (on behalf of Vodacom) called me to offer a reduced rate for my Vodacom account to which I agreed, they assured me all would be as it as in term of my available minutes etc. To my frustration a few days later my account was "out of credit" and I needed to recharge (which I have never had to do, as I receive a monthly bill for the amount of call time used). I have logged several complaints with Vodacom but to no avail, each time I am assured this issue will be resolved and the new terms will be reversed. This morning I received an sms stating the my query/dispute was "invalid". I want this resolved soonest as it has affected my business and I am frustrated at all the wasted time spent on this mismanaged agreement.