Depends on how the phone gets it's IP address.
If it's done via DHCP, you need to add the string to the 176 or 242 option (depending on model of phone).
If it's statically addressed, hit MUTE and dial R-E-S-E-T (73738) and hit #. You want to reset the settings and reboot the phone. Then on boot up hit * to program when prompted and enter the IP settings. One of the options is to adjust the VLAN Flag...