OK, if you're not able to program from a PC with the WinSPM software, you may be in over your head.

Try this from your Operator Phone/Programming Phone:

Menu
System Program
Start
Extensions
More (the Right Arrow Button)
Remote Frwd
(If you see RCF Extensions, press that)
Dial the Extension Number of the phone that won't forward
Enter
Back
More again
TrkTransfer
Dial the Extension Number
Enter
Exit

This will allow the phone to do Remote Call Forward. You program it by going off hook on the phone, dialing #33, 9, the remote number, and #

See if this will make it work!