The internal extension number in our office ranges from 800-900. Which was exactly that way it is when we were still using analog PBX lines. We were glad that we didn't have to change extension numbers for different departments.
However, there are a few analog phones/Fax machines we would like to keep using but we cannot seem to change the extension number from the default MyPBX extension series of 600. The extension number field is grayed-out for this option (as shown in screenshot).