Clearly this is too late to help you, but since this is showing up on Google and there are about a thousand views, this answer might be helpful to another.
Once you access the Voice Admin menu and provision your device for your VoIP provider, access to the Voice menu is subsequently "Locked".
- Your VoiP provider's system has set a new unique password for the Voice menu once the provisioned device has synced with your providers system.
- You should be able to get that password from the control panel on you VoIP account profile under "equipment" or a call to support or an e-mail request should garner the password.
- The Login will be "admin" and the password provided will get you back into Voice menu system.
You will also need this password if you want to reset the Voice features back to factory defaults. Because of this system it appears that a WRP400 cannot be unlocked by merely using the reset button.