Point your DID to the actual extension of the person. Create a phantom device and forward it to the outside number you wish. Create a Call Routing Application. Give choices to call the mobile or leave a voicemail. If they choose the mobile, have it go to the phantom. If they choose the voicemail, have it go to the person's mailbox. When this person wants the calls forwarded, they activate FWD on their phone. This would affect all calls going to that extension.

Or, point the DID to the phantom device and allow the user to program the forwarding (cell phone or extension) by remoting into the phantom device (359).