See existing thread
Optus can be a bit hard to get through sometimes but I've never had to wait more than 5 minutes in LIVECHAT. You are being scammed so you should take the time to sort the problem properly if the money involved is worth it (the scam often works because its too much hassle for the relatively low amount)
1. Send STOP to the SMS number that is involved
2. Contact Optus and get your Premium SMS limit set to $0
3. Request Optus refute the charges (however they will usually only do this for the current bill, but you might get lucky)
4. In the future, if you get more dodgy SMS subscriptions Optus are no longer allowing them to charge via Optus (so you can send STOP and just ignore them as they can't bill you anymore.