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