With respect to the extension number, Changing it is as simple as going to form 9 and typing in the new number over the old one.
Digit translation takes several steps.
1 - Form 2 - enter digits for feature access code for digit translation
2 - Form 15 - Enter FAC entered above as the N digits to be inserted (if you already have ANYTHING in the N field DO NOT CHANGE IT)
3 - Assuming you get this far, i believe form 55 is the Did translation table. enter the digits for DID's you wnat to translate (without the FAC code) and enter the destination for Day, night1 and night2.
Note: you do not have to enter all of your DID's. only the ones to be translated. If the DID is not found in the table it will be dialed AS IS.
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Occam's Razor - All things being equal, the simplest solution is the right one.