Use 'Call Forward Forced'. If you have VM set up and a forward to VM of some kind set up (doesn't have to be active) just use it if you have one of these phones 5020 IP Phone, 5220 IP Phone, 5220 IP Phone (Dual Mode); 5224 IP Phone, 5235 IP Phone, 5324 IP Phone, 5330/5330e IP Phone, 5340/5340e IP Phone, 5230 IP Phone.
There are some caveats as well so peruse the help files under Call Forward-forced.