Another reliable way to do this would be as follows:
In active directory ("users"), it is assumed you have a user called Some Name with an email address of
[email protected]
You could right-click on the "users" folder on the left, and select NEW then CONTACT.
Name the contact something like SomeNamesCellPhone and give it the email address of the cell phone.
Your other users can then pick either
[email protected] for sending emails "to his work station"... or they can select SomeNamesCellPhone to send it to his cell instead.
Optionally, you can then set the user account itself to always forward its incoming email directly to the cell phone contact you created. To do this, you would likely need exchange installed. Do you? If so, right-click the user, properties, exchange advanced, delivery, forward to: pick the contact you made above.
Note that you can ONLY forward from active directory to other active directory objects. The trick is, the CONTACT you created (with external email address), is an active directory object, so forwarding off-site works great.