You probably want to wait until one of the Executone voicemail pros chime in to use the following so you don't mess something up..but this probably will work..it's only for ALL calls - int & co.

7*3 - Forwards ALL intercom & co's in DAY mode
7*5 - Forwards " " " NIGHT mode

7** - Cancels ALL call forwarding in DAY mode.
7*9 - Cancels " " " " NIGHT mode.

Write these codes down and wait for a V.M. pro to chime in.