My NEC DVL(XD)Z-Y (BK)phone has a red blinking light for reception button. No one knows the mailbox code since the receptionist was replaced. How do I reset code or stop light from fast flashing red?

SOURCE: features

Not sure how you set this feature but here is the solution:

To set DND you would have pressed your feature button, 60,and then feature button again (if you were forwarding to another extension you would have had to enter an extention after 60 and then the feature button. This sets your DND or call forward.

To eliminate press feature, 69 and then feature again this should cancel the call forawrd or do not disturb.

SOURCE: how do you change the time on a nec phones dth-8d-1(bk)tel

To Update Time: From operator telephone with receiver hung up; Press feature key; dial 9,#; enter hour and minute i.e. 0135; (press recall key to change from am to pm); press feature key to save and exit.

SOURCE: NEC Dterm80 ring tone

To here ring types hit speaker then 711, then 1 for internal rings (select 2 for outside ring selection) then 1-8 the ring type.
To set ring type hit speaker then 720 (follow all the steps above).

SOURCE: I need to change the

you probably have an ext unit please try the following steps
To set the time, Press FNC then 9 then # then set the hr and mins using number keys Use RECALL button to set AM & PM Press FNC again to set.
Good luck and best regards
John

SOURCE: change the time on our NEC DT300 Series

What model of NEC phone system do you have? Go look in the phone or data room and look at the PBX. You might also be able to press Feature 3 to get some information which would be helpful to a tech. If it is an SV8100 try this:

To set the system Time:

1. Press

Speaker.

2. Dial

728.

3. Dial two digits for the hour (24 hour clock, 13 = 1:00 PM).

4. Dial two digits for the minutes (00~59).
5. Press

Speaker to hang up.