From aa-asterisk.org.uk wiki
NSN is short for "National Significant Number".
This is the telephone number shown without the initial 0 trunk code or +44 country code.
Specifically, it is the SABCDEFGHI digits for a 10-digit NSN, or the SABCDEFGH digits for a 9-digit NSN.
One minor issue is that while Ofcom refer to 10-digit NSNs using SABCDEFGHI terminology, BT use SABCDEFGHJ instead, avoiding issues where the letter "I" is mistaken for the digit "1".