From the NEC DSX forum site:

Off Premise Call FWD cannot be changed remotely at all.

If you use the Intramail External Transfer feature instead of the Off Premise Call FWD feature you can set up several Routing MB’s with Dial Action Tables using different phone numbers. Then the user can call in remotely and use the Schedule Override feature to select the Routing MB.

The External Transfer feature is explained in the Intramail section of the DSX Software manual. You do a UTRF to a system speed dial bin. (UTRF to #201P) Then set up the speed dial bin to select a line group and enter the phone number. The only other required entry is in Line Setup. You must enable Tandem Calls on all lines.
Schedule Override can be done from any System Admin mailbox. Dial 72 (SA - For System Admin) and 6 (O – Override) then follow the prompts to activate and deactivate Schedule Override.

"I didn't do it, and I won't do it again"