Unhappy ex-client

I called the MTN Call Centre on 02/09/2020 to get a settlement amount for the contract I had with them. The consultant I spoke to provided me with a figure which I double and triple checked. I then made the payment on the same day. I am then billed an additional amount on 29/09/2020,which I've been querying since then. I took to Facebook with no feedback, hoping to get some sort of resolution on this platform because they just aren't showing any interest in addressing/resolving my query.