@LingziZhang Yes Crowd is a public forum and not a direct support point for Optus.
Note: Yes Crowd cannot access or view customer accounts/orders.
If the number was issued by Optus it should be possible. Phone numbers are stored for 6 months and then recycled. If the number was ported to Optus the number reverts back to the issuing Telco but I do not know the exact time frame whether it is immediate or after 6 months.
Option 1 Please message Optus Support 24/7 from the My Optus app. If you do not have access to the My Optus app you can use social media to contact Optus Support, please send a PM to Optus on Facebook or a DM on Twitter and include in the message you do not have access to the My Optus app.
Option 2 If you are not in lockdown and practical visit your local Optus Store or call them. The Optus Website the last time I checked had a direct telephone listing for the local stores.
Option 3 Contact Optus Sales 1800 780 219
You will have to sign up for a current plan.