All comments are fully public and Optus staff can't see your accounts or fix any specific issue you may have. You will need to contact Optus directly (phone, facebook, etc) and there is likely a long wait.
Thanks @petergdownload for your information. I saw Dan_C comment how he can disable them via back end and that is why I try the private message.
Getting frustrated with the service centre who does not know how to resolved the request as many Optus customers encountered with this issue. They do not pass it to the correct person to action it or not trained properly to resolve this call waiting issue.
Manage to diable call waiting by logging on to MyAccount. Pick the correct account if you have multiple services with Optus. Services - Home Phone - Setting - Voice Call Setting - Call Waiting - step 1. Call Waiting On then save. step 2 Call Waiting Off then save.
When you are in a call, second person call will get a message you are busy and end with engaged tone.