This can also be done from the web portal interface.
On the Phones dial pad, Dial *72 followed by the number you want to forward your number too.
Example: You want to forward your extension to another extension. Your Extension is 1001 and the extension you are forwarding to is 1003
Dial *721003 The extension 1001 is now forwarded to 1003.
Example: You want to forward your extension to and external number. Your extension is 1001 and the number is 214-555-1212.
Dial *722145551212 The extension is now forwarded to external number.
To remove call forwarding Dial *73 by itself and the phone will inform you that the forwarding has been removed.