TIO is only interested in if the contract has been properly observed. It sounds like it has and if you halted a payment plan then usually that then just reverts to you again owing the full amount immediately. Its good you are educating yourself on the tools etc involved as you have some responsibility to be able to use them. If $900 of yours has gone missing then you need to work out where it has gone yourself. Do you have confirmation of transfers from your bank account to Optus? Reciepts? When you have this information you can contact the billing department and ask them to chase it up.