"MA SECDEF" is probably the SECDEF's Military Assistant:

> In the United States Department of Defense, a military assistant is a military officer serving as aide to very senior civilian (typically a presidential appointee in Office of the Secretary of Defense or in the service secretariats)[1]

I'd guess that "MASECDEF QTRS" is that officer's "home phone" (at their living quarters).

Based on the office phones I've used, I'm guessing that the buttons with the 7-digit phone numbers are just used to select phone lines that this phone can make/receive calls on, not speed-dial numbers. I'd also guess that they're on an internal military phone network that's not directly reachable from the regular phone system.

[1] https://en.wikipedia.org/wiki/Military_assistant

You get to learn about the the Defense Switched Network[0] today. The US military does have an internal phone network, but the unclassified side can be reached from the normal telephone network just fine. One of the fun things is that if the network is busy, certain phones can do priority calls and kick lower priority calls off to get the bandwidth freed up. The red buttons to the right of the key pad allow that phone to do that.

[0]https://en.wikipedia.org/wiki/Defense_Switched_Network

> Based on the office phones I've used, I'm guessing that the buttons with the 7-digit phone numbers are just used to select phone lines that this phone can make/receive calls on, not speed-dial numbers. I'd also guess that they're on an internal military phone network that's not directly reachable from the regular phone system.

Yeah, listing both lines there also might allow this phone to "barge" in on an existing call on one of those lines. If one is in use, it might show up in a different color, allowing the user to select the unused one if they wanted a new line, or the used one if they wanted to join in on the call in process on that line.

The plain numbers are likely the incoming lines/numbers. Like an indicator to show whether or not 'line 1' is ringing.