Well, if you have Call Forwarding enabled in your class of service you can do the following to call forward external:

1: Create a system speed dial bin with the number that you want to forward to. Using the superkey select call forwarding always. Program the destination as the speed dial feature access code and the bin number. (For example, if 4 is the feature access code for speed dials and 66 was the bin number you would enter 466). Press save and then make sure that you turn it on. You can check this by going back into the call forward setup. If you see a * behind the destination it is enabled. If not use the softkey to turn it on.

2. This is a better way for the end user.
Program a speed dial key on the set. Program call forwarding as above but when propted to enter destination, press the button that you programmed and it will enter the number for you.

You cannot just enter a any number into the call forwarding for security reasons. This makes it a little harder for the end user to forward their phones where ever using up trunks.

You could also program a call forward always key on the set and toggle the forwarding on and off at your pleasure.

This feature will not work if the system is not programmed correctly. One way to check is the call the extension internally. If it forwards to your external number your programming is good for destination and what your set is allowed to do. If it does not work when you call from the outside, there is a programming issue with trunk to trunk transfer or device interconnect restrictions.

Hope I didn't lose you in my explanation.

Simon