Open the Phone app
Tap Voicemail
Check the Deleted Messages or Deleted Voicemails section if available
Select the voicemail you want to recover
Tap Undelete or Recover
If it is not in Deleted Messages, contact your carrier
Restore from an iCloud or iTunes/Finder backup if the voicemail was backed up
Make sure Visual Voicemail is enabled
Restart your iPhone if voicemail is not showing
Update iOS to the latest version
Reset network settings if voicemail still does not appear