You wont be able to get a transcript of a voice conversation thats not kept at all (and can have numerous legal implications depending on each states recording rules)
And I would say its very similar to SMS in regards to the content of the SMS not being recorded.
You can request an itemised bill that would show incomming and outgoing calls and sms but not what is in them.
Do you mean of your conversations with Optus? You can request those on an individual basis and they may be available.
Otherwise, thankfully, Optus do not conduct surveilance on our calls and SMSs (That's googles job ).