On my iphone 13 mini, I can't un-delete a voice-mail. help

How long ago did you delete the voicemail? Did you open the Phone App, tap Voicemail and scan ALL the wy down to the bottom of the page and tap Deleted messages? If the message is there, tap it and see the red trash can with a line through it and tap the trash can to "undelete" the voicemail. If it's not there, Kurtosis12 is correct and you'll need to contact your carrier and hope they have a copy of it still. They are never really stored on your iPhone to begin with. They are stored on your carriers servers.

Is the voicemail in the Recently Deleted folder? If yes and when you tap it, there is no red trashcan, you'll need to contact your carrier as again, Voicemail is a carrier feature. Your carrier may have a different method.

Voicemails are stored on your cellular carrier's servers. Contact your carrier and see if they can retrieve it.