MTN over billing

I upgraded my contract via telephone. Mtn called me to say I'm due for an upgrade of which I was aware of. Come month end I get over billing. Tripple the amount I was expecting to pay for. I call Mtn and they tell me there's nothing they can do, I have to call the agent. Did I hire the agent to upgrade my contract??? I upgraded via telephone because I trusted Mtn as my provider. And that person provider all the details that I know only Mtn had. But why the over billing ????