Its a scam. Optus finally closed it off after a decade or two last month. You need to:
1) LIVECHAT Optus
2) Request Optus refute and refund the charge
3) Request Optus set your Premium SMS limit to $0
Some staff will still say its your job to sort out but most should sort it for you. A refund should be on your next bill.
Otherwise a mod here should be able to sort it @Dan_C