From aa-asterisk.org.uk wiki
NDC is "National Destination Code".
For geographic telephone numbers, the NDC digits are the geographically significant leading digits of the NSN, but shown without the initial 0 trunk code. The NDC digits can be omitted when making a local call within the same area.
The NDC is, in effect, another name for "area code", previously called STD (Subscriber Trunk Dialling) code. Although many people refer to the combined 0 trunk code and NDC together as being the "area code", technically this isn't quite correct.
The NDC can be two to five digits long. A list of different length "area codes" is shown in the table on the 01 numbers page.