Try, Feature, hold, #, 0, * from telephone ports 01 or 02. You can get a programming manual at
FEATURE, #, 0, *, 8974 is a back door from any telephone port. Not published, but the system is no longer supported by NEC.
Once in program mode, press line key 7, then line key 1. The bottom line shows cabinet number (1), slot number (01) : xyz (current card assigned), starting port number = 01, above that is additional port numbers if it's an 8 port card. A line key should be lit to represent the card type. (ESI = station card). Use * or # to move the cursor left or right. Go to the slot number and input a vacant slot number.
Line key 1 lit = no assignment. Press line key 9 on a 16 button phone to assign a station card. Press TRF to write it. Press line keys 7 + 1 again to refresh then move the cursor under the slot number and enter the slot you just assigned, then note the port numbers. You will need that to assign a station number.
Press ANS key to back up in program. Press line key 4, then on the key pad dial 10 to get to extension numbering. Press CNF key to advance station ports, and look for an unassigned number. Careful, you can double assign number and get into trouble. Assign an unused number to the card/port just installed.