If you've ever found random charges for apps or games on your mobile phone bill that you don't remember signing up for, this could explain it.
Optus is in trouble with the Consumer Watchdog after admitting to opting in on their customers' behalf for direct carrier billing.
"When consumers click on to various apps that give them ringtones or games or horoscopes, they can be held to have subscribed with just one or two clicks, and without having to enter credit card details or anything like that," Mr Simms said.
"You could have just clicked on the site, you could have clicked to view it, maybe you clicked to buy (but you may have thought that was the beginning of the transaction), or the mobile phone could have been in the hands of a child who wasn't aware of what they were doing."
This behaviour has been described as appalling by ACCC Chair Rod Simms, who told the Hit Newsroom that it is now before the courts, with Optus facing a $10 million fine - as well as possibly having to repay customers who are out of pocket.