Open the Phone app
Tap Voicemail
Scroll to the bottom and tap Deleted Messages
Select the voicemail you want to recover
Tap Undelete or Recover
Check your Voicemail inbox to confirm it was restored
If Deleted Messages is empty, contact your carrier
If the voicemail was permanently deleted, ask your carrier if they can restore it from their system
If you have an iCloud or iTunes/Finder backup from before deletion, restore the backup to recover the voicemail
Make sure visual voicemail is enabled on your iPhone
Restart your iPhone if the deleted voicemail does not appear after recovery