$900 is equivelent to about 80Gb of mobile data. Its not an amount that's easy to inadvertantly download. Do you consume a lot of TV on the phone?
Unfortunately Optus have a policy of unlimited topups for data ($10 per 1Gb) which means soem people are going to get really stung. The SMS notification requirement is also a bit wishy washy (it can be several days before they have to notify you)
Do you have an Android or Windows phone? If so you should set the phone to warn and cut off data at set amounts each month so that you're not relying on Optus to warn and limit you. (Something from here on out)
That said. It sounds like somethings gone awry. You should be able to see what apps have consumed you data on your phone. Check the settings. Optus should also be able to tell you broad categories.
The "other person SMS" seems germaine. Wires are possibly being crossed. If Optus can't help then make a compaint to the TIO ombudsman and see what they can find out on your behalf.
Hey Maddie, I can look into this for you, so sorry for the run around you've already had to do for assistance. Please send me a private message with your number, name and DOB to discuss.