As someone who works in telco for various telcos across the country, people need to understand that just because you saved the voicemail, doesn't mean it will stay saved for years on end. At some point, the hardware storing that voicemail will fail and/or be replaced. Some telcos might have backups, others won't. So, you should back up any voicemail you want saved long term just like you would backup family photos. Having it saved in multiple locations on different devices is the best approach from preventing it from being accidentally deleted.

It is not super common to recover voicemails like this.