© 2013 Brice Center (first posted 8/7/13 ► last updated 8/7/13)
This method could be used for phones other than iPhone (even land-lines), but my instructions are tailored to the 3-way calling method on iPhone.
If you already use Google Voice, you'll need to do the following to get ready
Turn off Google Voicemail direct access on the phone you will be using (in this case my iPhone)
Tap + Add Call when it become available (I had to wait 10 seconds until it lit up)
Dial your Google Voice Number and tap Call
The Google Voice call will ring, and then Google Voicemail will pick up and start playing your greeting. BEFORE you hear the Google Voicemail beep, tap 1 (on Verizon) to start playing your voicemails on the Verizon system. If you wait until after the Google Voicemail beep, pressing 1 will actually end the Google Voicemail prematurely.
When the voicemail is done being recorded, tap End Call
Undo the changes made to Google Voice Settings to re-instate your Google Voicemail on your Mobile, your Direct Access, and your Forwarding phones (reverse the prior instructions).
Go to Google Voice on a computer. Find the voicemail in your inbox. Click "more ▼," then click "Download." Now you have an .mp3 of your old voicemail. If you want to trim the prompts before or after the actual voicemail, use the great free program MP3DirectCut to losslessly trim your mp3.