Dan231,
If you are not that familiar with the system then maybe doing DEE is the best way for you; that is if you have DEE on your system.
Create a phantom, but don't worry about doing the forwarding portion. Now, create a User and select that Phantom as the Main Number. Then go under Associated Destinations and set the Mobile to the number you want it to go to. You can set the Main extension to Inactive, but definitely set the Voicemail to Inactive. This way you can also send the call to the number you want and easily change it when necessary. Either way you are taking up 2 lines on your system unless you have the ISDN 2 B-Channel Transfer enabled on your system and then doing the forwarding, as done previously, is your best option.
Thanks,
TE