Open the Phone app
Tap Voicemail
Scroll to the bottom and tap Deleted Messages if available
Find the voicemail you want to recover
Tap the voicemail
Tap Undelete or Recover
Check the Voicemail tab again to confirm it was restored
If Deleted Messages is not shown, contact your carrier
If the voicemail was permanently deleted, ask your carrier if recovery is possible
Restore your iPhone from an iCloud or computer backup if the voicemail was backed up
Check Visual Voicemail settings and network connection if voicemails are not appearing