Using the OBi phone interface:
Is there any way to delete a Voice Mail, after you listen to it?
Or is the only way to delete GV voice-mails, is only via the web interface?
Why would GV not allow a voice prompt (via phone interface) to delete a voice-mail after listening?
You "archive" the message via the phone. You can only delete it online.
Google has a philosophy that you should never need to delete anything just to make space. They promote archiving.