Directory number
From Zenitel Wiki
A Directory Number is the number to dial to reach a station or a feature. Default directory numbers after autoload are 101-499 and 5000-5152. The default number can be changed to any 2-8 digit number starting with 1, 2, 3, 4 or 5. Numbers with first digit 6-9 are reserved for features. If these numbers should be used, the corresponding feature code must be changed.