You can create voicemail forwarding on any extension by doing the following:
Log into the extension (via user portal) that will receive the mail and forward it
Go to Voicemail and Fax and choose -> Voicemail Options
Go to the Forwarding tab
Enable forwarding for an extension, multiple extensions or extension groups.
You can manually forward a voicemail from the web interface as well:
Log into the extension (via user portal)
Go to Voicemail and Fax and choose -> Mailbox
Selec the voicemail you want to forward
In the top right, there is a section to select an extension. Once you have selected the extension you want to forward the voicemail to, click the icon with the green arrow to forward the voicemail.
You can manually forward a voicemail from the phone itself:
Press the voicemail button on the phone to bring up Visual Voicemail
Selec the voicemail you want to forward and then press More...
Press the Forward button and select the extension you want to forward the voicemail to