That seems to happen more often than it should. Optus are suppose to register your existing number with the NBN so NBN can set it up on their system when you are activated.
Try ringing NBN and asking what number is showing on their records. Ask them if that can be changed by request from Optus now.
Unfortunately I'm not sure if anyone has managed to get a number back once gone. The next point is to get someone at Optus to request the NBN update the number involved. Depending on who you get to help ... ?
Hey @Somap, generally speaking we would issue you with a brand new number unless you do specifically request to keep your existing one.
I do hope that this has since been resolved for you! If not, please chat with us via Live Chat here HERE so we can help out 🙂