The "numbering plan" means your phones are either 2-digit numbers, starting at 10, 3-digit numbers starting at 100, or 4-digit ("set up space"

starting at 7100. The smart money puts the system in set up space, and then renumbers the extensions one by one to get the final numbering scheme.
You can figure out how your's was set up by printing the dial plan, and looking at unused extension ports to see how they are numbered.
Below is a cut-n-paste on a cheat sheet on renumbering that I made up for some customers:
Renumbering Extensions
Renumbering Extensions allows 2 (or more) telephones to switch places without physically changing connections at the Control Unit.
Renumbering is a three-step process. First renumber the 1st extension to its "set up space" extension number, then renumber the 2nd extension to the 1st extension number, and then renumber the 1st extension from its "set up space" extension number to the 2nd extension number. The "set up space" extension number is the 4-digit number in the "set up space" column of the system planner. Alternatively, you can find the "Logical ID" of the extension, and calculate it by adding 7099.
Example: Swap Extension 100 and 101. Look in the system planner and find that Extension 100's set up space number is 7100. Renumber 100 to 7100, then renumber 101 to 100, and finally renumber 7100 to 101.
After renumbering, you also need to re-label the extensions, as they will still have their labels from the old extension numbers.
Renumbering Procedure:
-Menu
-System Programming
-Start
-System Renumber
-Single
-Extension
-Dial the Old Extension Number
-Enter
-Dial the New Extension Number
-Enter
Labeling Procedure:
-Menu
-System Programming
-Start
-More (or ->)
-Labeling
-Directory
-Extension
-Dial the Extension Number
-Enter
-The current name will be displayed
-Use "Backspace" to remove the current name
-Enter New Name (maximum 7 characters)
-Enter
Determining the "Logical ID" and Set Up Space number of an Extension:
-Menu
-Maintenance
-Port
-Station
-Dial the Extension Number
-Enter
-Status
-The screen will show the current extension number, the "Logical ID" and the "Slot/Port"
-Add 7099 and the Logical ID together to get the Set Up Space Extension Number
-Exit