Currently getting through to Optus can take a VERY long time. Call centres have been closed. Optus are also generally prioritising service issues (no connection or phone) and financial hardship. Its expected that Optus will re-credit things like late fees or backdate cancellations if you can't get through until later on. Depending on the issue you can try the following:
Slower: Write to the official complaints address and your requirement will be dealt with in due course (If you want to cancel for example Optus will backdate charges to the dates in the letter)
Customer Relations Group, PO Box 306 Salisbury South 5106.
Don't press 1 and hold the line, you'll either be on hold and get through to someone or get the standard IVR to enter your service number and follow the prompts and press 3 to cancel. Times can vary from a few minutes to many hours.
Alternatively: Leave a message in the Optus Mobile App (If possible) or try Optus Twitter or Facebook sites.
I had the same issue with them. I would call and they would "redirect" me to the cancellation team and the call was "dropped". Every time it transferred, the representative couldn't hear me on the other end of the line or it just dropped automatically. This happened 7 times in a row.