No probs, this is just a public forum not a helpdesk. Its likely a MOD will pop in and offer to try chase things up further though.
Your issue appears pretty low level (by that I mean someone is going to have to dig into the guts of the billing sytem to try figure out what is happening) Hopefully you can reach someone who can do that.
In the mean time I would be concentrating on the side you do have more control over (as you have been). If it is some issue on the phone then there's nothing Optus could (or should) be able to do - Everything is working 100% on their end. I can only suggest turning off the phone for a few days (over night?) and recording such times and then comparing to any data usage you are getting hit for. If the phone is off and you get a bill then its right back in Optus' court.
Good luck with your investigations