temple,
This is not a 3300 so you can't do it the same way you would on a 3300. Personally, what I would do is just reject calls to that DDI by making it a restricted number to be dialed which will force them to start using the extension number or call via their cell phone.
If you are really wanting to make this work the only way I can think of is fake the system out in thinking that the call is leaving the system whhn it isn't. You can do this by taking your built-in Loop Start Ports and physically connecting them to your built-in Single Line ports; the more the merrier even though you only have 4 of each.
Once that is done then create a CO Trunk Group for the 4 built-in Loop Start Ports; don't use the default PP011. Then have the Single Line Ports setup as House Phones to dial the Configuration Assistant. If that doesn't work then try setting up a System Forward Immediate to go to the Configuration Assistant. The reason I say it may not work is the Configuration Assistant may identify the Single Line Ports extension as the extension that is being modified by it.
Then go into Facility Groups and create a new one for your CO Trunk Group with the built-in Loop Start Ports, but don't have it use any of the Dial Rules so nothing is sent after it goes off-hook. Then go to your Route Groups and set it to use the Facility Group that you just created and set your Dial Pattern to be the DDI that they are calling; viola.
This way when the number is dialed it will go to the Route Group that identifies with the built-in Loop Start Ports. The Loop Start ports go off-hook and dial nothing, but the Single Line Ports will send it to the Configuration Assistant. You can only have as many people using it as your have Loop Start Ports connected to Single Line ports, which by default is only 4. This is assuming that you are not using any of those ports already.
Thanks,
TE