MTN STOLE FROM ME!

I am an MTN client for over 10 years and for the first time since i am unhappy with the manner in which i'm being treated. I recently upgraded my contract as per normal every 2 years over the past 10 years with no changes made to the specifics of the contract. This time around which has not happened with the previous upgrades my airtime balance went from R2,283.00 to R0. If i was informed that this would happen / made to sign an addendum of sorts i would not have proceeded with the upgrade and would have considered moving to another service provider. I have contacted the call centre 4 times and visited the service centre on 2 ocassions and was told that the back office would contact me regarding the reinstatement of my airtime balance prior to the upgrade. I have yet to hear from MTN regarding my query.