That is the catch with call forwarding, that the calling number is teh extension being forwarded.
I see two options:
use twinning, the original calling number will be sent
use a shortcode
create a time profile for after hours, and in the Incoming Call route for after hours route it to the external number, selecting the passthough the CLI
Refer to
http://marketingtools.avaya.com/know...characters.htm