These are very common problem. Some of us by mistake falls in this trap. So don't worry just write a letter as sample is give below and relax.
XXXX (COMPANY NAME)
XXX (Address -City etc)
Sub- Illogical data download showing on my SIM (For the month of September-2012) till 18-09-2013.
Respectfully I want to say that I checked my SIM internet data download on 19-09-2013 and found an illogical data download (4528060 KB i.e. 4.5GB approx). My SIM is under XXXX plan. My SIM is under GPRS PLAN-89 (or what ever your internet data plan). As far as I know there is maximum data download limit is 2 GB under this plan.
I don’t know how it happened. I didn’t get any alert message on my mobile when my download limit was crossed. Fortunately I checked my account on 19-09-2013 and noticed the unusual download.
My mobile no-XXXXXXXXXX
I therefore request you to kindly see the case and do the needful to reduce the future illogical bill and oblige.