That usually means that Optus haven't completed the porting process correctly. They need to put a redirect entry into the phone numbers database to redirect incoming calls/messages to the Telstra network. You will need to ask Telstra to contact the Optus porting team to get it rectified.
The easiest way at the moment is to send a message using the My Telstra app on your mobile (blue icon, bottom right of Get Help screen). If the bot doesn't understand the query, type "Speak to a consultant" and you should be transferred to the queue to chat with a human. It can take a few hours to get an initial response and you will receive a notification through your mobile when they do.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.
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