I have a Samsung Galaxy, but maybe they work the same. This is what I do to retrieve a voicemail that I accidentally deleted:
1. Open the voicemail icon
2. Go to the very top of the phone and click where it reads Inbox.
3. A small scroll down menu should appear with Inbox, Trash, SD Card.
4. Click on the trash icon and you will be able to retrieve whatever voicemail you had deleted.
Hope this helps with your DROID as well!