Physical number
From Zenitel Wiki
The number series 101 to 499 and 5000 to 5152 are default directory numbers for all AlphaCom exchanges. The directory numbers are connected to physical numbers in the range from 1 to 552, where the physical numbers are given by the subscriber point on a board in a certain position in a defined module. Since the card positions 2 - 7 are not used in AlphaCom E20 (and 80+), the corresponding default subscriber numbers are missing. The directory numbers can be changed by using the AlphaPro programming tool.