They keep billing me more and more!

I requested an upgrade several months ago, the agent incorrectly upgraded the wrong number.

Since then I've been double billed for the upgrade, and every time I speak to someone to resolve it, I'm billed even more the following month. This is getting out of hand, and I'm about to go the ombudsman, because MTN claim they can't help me, the agent who processed it has to fix it, and that's clearly not working for me.

The agent was Nishen from MTN La Lucia. If I don't recover the extra I've paid by next week, I'm going to do whatever it takes to get off of MTN.