Just figured this out: I was missing the CO group access code for the all the lines. I found it in a programmer guide online, it is as follows:
To activate off-net forwarding:
1. Dial [603] on the dial pad,
-or-
Press preprogrammed CO OFF-NET FORWARD button.
2. Dial the CO group access code of the group to be forwarded:
[801-823] = CO Group 1-23
[824] = All CO Groups
3. Dial the speed bin number that contains the number where calls are to forward. A
confirmation tone sounds.