Getting back a voicemail I deleted on iPhone

If there's no Deleted Messages section at all, your carrier probably doesn't use visual voicemail and the message lives on their server. Dial your voicemail the old way, some systems keep deleted messages for a bit. Restoring an older backup can sometimes bring it back, but that wipes everything newer on the phone, so only if it really matters.

Phone app > Voicemail, scroll to the very bottom. There's a Deleted Messages folder, open it and hit Undelete. If you already cleared that folder, ask your carrier, some keep them a while.