@DrossGoogle "recall last called phone number" and see if the answers work. Be aware that these are for the old POTS system, not NBN.
Add NBN to the search and have a read. I do not have a landline via POTS or NBN, so I cannot try either of these.
Also there are Optus pages in the link that give the info like the following:
Hopefully this is allot better info than you got from Optus's response yesterday.