That depend.....
There are two ways to forward.
1- through the programming of the PBX. You'll need to login, go to "All forms" then "Change Attributes Assignment", edit the phone and change Call Rerouting 1st alt to 1, Call rerouting 2nd alt to one, Day, Night 1 and Night 2 to 1.
2 - you can forward the phone through the menu on the phone. Hit the super key and then say no until you see call forwarding and then hit the yes button. Any call forwarding you see, turn off.
Ralph
Note: I'm making some assumptions here. Some of this depends on what type of phone you have and how your vendor programmed it.