All in NUMBERING PLAN. ROUTE GROUP is where Automatic Route Selection inspects the number you dialed to determine how to route it. Based on the ROUTE GROUP call is then sent to a FACILITY GROUP, which tells it what to do to the string (add digits, drop digits) and then what trunk group to use for the call. So, based on what you've said, I'd create a new ROUTE GROUP with 1 dial pattern only...that pattern will be 5555555555, for example, and that's the number you'll forward your phantom to (be sure phantom has ARS, code 92000, programmed in OUTGOING field of ASSOCIATED EXTENSIONS). Then do a manual forward of that ext to 85555555555. Then in NUMBERING PLAN/DIAL RULES create a new rule that says only 8, desired outside number, P for pause (maybe 2) and acct code. Create a new FACILITIES GROUP that uses your new dial rule only and uses whatever trunk group you want to use, and go back into your ROUTE GROUP and select this new FACILITIES GROUP as the only facilities group. Also now drag your new ROUTE GROUP to the top of the list of ROUTE GROUPS, above local etc, so these calls will fall out before they get to local or LD.
From any phone dial 5555555555 and see if calls go to the right outside number and you don't need to punch in code. If so, dial the phantom ext, expecting the same result.