Retrieving deleted voicemail greeting on Telstra iPhone

hi, i recently upgraded from a pre paid number to a plan with Telstra. I’ve kept the same number but my voicemail greeting has been deleted. It was a message from when I was a kid and my Mum really wants a recording of it. How can I get it back? Any help would be appreciated thank you

[Re-Titled by Moderator]

Original Title: Voicemail Greeting retrieval