This happened before and have been ongoing for quite a lot of people. I myself had been billed wrongly on data usage due to change of plans so I'm not surprised for something like your case to happen as well.

What you can do is, be consistent and keep calling up the maxis call centre (every 1-2 days?), keep track of the calls (time that you called and what you have told them) since there's recording going on during the conversation. Once they manage to escalate this to a supervisor/manager/whoeverthinksthattheyhavehigherauthority, they will search back the recording (yes every call are recording whether u like it or not) and get a better understanding on the issue. Do becareful with Maxis terms, when this case drags on for weeks and you pass the billing payment date, they MIGHT (<-- read pls) cut off ur line and saying that you didn't clear your bill (Clearly, Maxis internal team doesn't communicate to each other at all).

Seriously, I'm not bad mouthing Maxis nor i dislike Maxis (8 years with Maxis now and I'm happy with their services and products) but human errors plays a huge role in escalating and also getting problems fixed.

Just my 2 cents anyway, take it or leave it.

All the best to you TS.