Been down this road before!
Many years ago I was with a carrier (I can't be 100% sure which one as it was at work and don't want to name names incorrectly) who sent me a bill for ~650MB of data used in a day.
The device was a 2G (No, not even edge) device (M2M device) connected via GPRS which had a theoretical maximum of 48Kbps (yeah, like we ever saw that).
I tried to make clear that the theoretical maximum even if the device had been transferring data at 48Kbps all day long was ~520MB of data but they refused to budge and threatened to disconnect the entire businesses mobile comms if the bill wasn't paid.
The beancounters just paid the bill to keep the rest of the company running :-(
From my own experience, I've not yet found a telecoms company I can trust...