Hi @Ianni, if your prepaid service is active you can Text 1 to 9999, you'll then be sent some details about your service, including the phone number.
You can also find your phone number by checking My Account on the My Optus App. For more info, please check out our How to find your phone number or username guide.
If you do require any further assistance with this, I recommend sending us a message via the My Optus App so that we can verify your details and assist further. You can also reach us by sending a message through to our Facebook or Twitter pages.