Majority of phone voicemails that are deleted on iPhones are not removed instantly once they are deleted. They are instead saved temporarily in a Deleted Messages folder, which allows the user to save them before they are deleted forever. One can access the Phone app, scroll to Voicemail, and restore messages in that case. Once this folder is cleared or the data is overwritten, then there is little that can be done. Visit here for more: https://datarecovee.com/how-to-retrieve-deleted-voicemail-on-iphone/