There are four different ways provided here to recover deleted iphone voice mail. https://www.tuneskit.com/iphone-recovery/how-to-recover-deleted-voicemail-on-iphone.html.  I used one method mentioned here and it works successfully. 

This topic is so hot these two days. I also met with the similar article here: https://gizmodo.com/yes-you-can-own-the-movies-you-buy-from-apple-kinda-1829031620, which mentioned TunesKit (https://www.tuneskit.com) can perfectly resolve the DRM issue related to iTunes digital content. It may be worth for try.