I found this handy how-to tutorial guide for "Talking to the Milnet NOC" and resetting the LH/DH, which was useful for guiding the NSA employee on the other end of the phone through fixing their end of the problem. What it doesn't mention is that the key box with the chase key was extremely easy to pick with a paperclip.

Who would answer the Milnet NOC's 24-hour phone was hit or miss: Some were more helpful and knowledgeable than others, others were quite uptight.

Once I told the guy who answered, "Hi, this is the University of Maryland. Our connection to the NSA IMP seems to be down." He barked back: "You can't say that on the telephone! Are you calling on a blue phone?" (I can't remember the exact color, except that it wasn't red: that I would have remembered). I said, "You can't say NSA??! This is a green phone, but there's a black phone in the other room that I could call you back on, but then I couldn't see the hardware." And he said "No, I mean a voice secure line!" I replied, "You do know that this is a university, don't you? We only have black and green phones."

Date: Thu, 11 Sep 86 13:53:45 EDT
From: Steve D. Miller <steve@brillig.umd.edu>
To: staff@mimsy.umd.edu
Subject: Talking to the Milnet NOC

This message is intended to be a brief tutorial/compendium of
information you probably want to know if you need to see about
getting the LH/DH thingy (and us) talking to the world.

First, you need the following numbers:
(1) Our IMP number (57),
(2) Mimsy's milnet host address (26.2.0.57),
(3) The circuit number for our link to the NSA
(DSEP07500-057)
(4) The NOC number itself (692-5726).

Second, you need to know something about the hardware. There
are three pieces of hardware that make up our side of the link:
the LH/DH itself, the ECU, and the modem. The LH/DH and the
ECU are the things in the vax lab by brillig; the ECU is the
thing on top (with the switches), and the LH/DH is the thing
on the bottom. The normal state is to have the four red LEDs
on the ECU on and the Host Master Ready, HRY, Imp Master Ready,
and IRY lights on at the LH/DH. If these lights are not on,
something is wrong. If mimsy is down, then we'll only have some
of the lights on, but that should fix itself when mimsy comes up.
Some interesting buttons or switches on the ECU are:
reset - resets something or another
stop - stops something or another
start - restarts something or another
local loopback -- two switches and two leds; you may need
to throw one or the other of these if the NOC asks
you to. These loopback switches should be distinguished
from those on the modem itself.
remote loopback -- like local loopback, but does something else.

The modem is in the phone room beside the terminal room (rm.
4322, if memory serves). It can be opened with the chase key from
the key box...but if someone official and outside of staff asks
you that, you probably shouldn't admit to it. It has a switch on
it, too; it seems that switch normally rests in the middle, and
there's a "LL" setting to the left which I assume puts the modem in
local loopback mode.

Now that you have some idea of where things are, call the NOC.
Identify yourself as from the University of Maryland, and say that
we're not talking to the outside world. They will probably ask for
our Milnet address or the number of the IMP we're connected to,
and will then poke about and see what's happening. They will ask
you to do various things; ask if you're not sure what they mean,
but the background info above should help in puzzling it out.

Hopefully, this will make it easier to find people to fix
our net problems in the future; it's still hard to do 'cause
we have so little info (no hardware manual, for example),
but this should give us a fighting chance.

-Steve