From the VG248 configuration screen:
MainMenu->Configure->Telephony->FeatureCodes
All you have the ability to program is the following:
Hang up last call
Transfer
Conference
Forward all to voice mail
Call forward all
Cancel call forward
Pickup
Group pickup
Redial
SpeedDial voice mail
SpeedDial 1
SpeedDial 2
SpeedDial 3
SpeedDial 4
SpeedDial 5
SpeedDial 6
SpeedDial 7
SpeedDial 8
SpeedDial 9
Depending on the CallControlMode you set (restricted, basic, standard, or feature) the user may not have access to all the features listed.