Line Redirection.
You will need a digital phone to set up Line Redirection. Once it is redirected though, you will no longer need it. You don't have to worry about routes and destination either: Line Redirection only requires an external phone number.
Line Redirection is pretty well explained in the System Coordinator Guide: Forwarding your calls > Line Redirection. Roughly, here are the steps you will have to go through:
[ol 1]
[li]Choose a M7208, M7310, M7324, T7208, T7316 or T7316E to set up Line Redirection.[/li]
[li]In programming, make sure the target line to redirect is set to Appear & Ring or Appear Only.[/li]
[li]In programming, make sure the Terminals & Sets > [DN] > Capabilities > Allow Redirect setting is set to Yes.[/li]
[li]Press Feature 84 to enter Line Redirection programming.[/li]
[li]Press the * key or the ADD softkey to create a new redirection.[/li]
[li]Press an Intercom key (if you have a PRI trunk) or an outgoing line (if you have anything else).[/li]
[li]Dial the external phone number where the calls must be redirected, then press the Hold button or the OK softkey.[/li]
[li]Press the target line button to redirect, then press the Hold button or the OK softkey.[/li]
[li]Press the Release button to exit Line Redirection programming.[/li]
[li]Call the redirected target line to make sure everything works properly.[/li]
[/ol]
Finally, you can remove the target line appearance. The system will handle the redirection, so the target line doesn't have to be assigned.
External Call Forward, which you used with an analog phone that was forwarded to some destination, only works as long as the ATA is connected to the system. That is because the set itself performs the forwarding.