head calls

Reader's Digest 6月号の記事 'Humor in UNIFORM' からの引用です。
In basic training, our drill instructor made it clear that everything we recruits used belonged to her. For instance, she referred to our foot lockers as "my trash," and the racks where we slept as "my racks."
One time when we were all whispering in the bathroom while making "head calls," she surprised us by yelling, "Why do I hear voices in my head?"
"head calls" は兵隊さん達の俗語の様ですが、気になるので調べます。
普通の辞書には "head calls" の項目はありませんが、俗語の場合に頼りになるのが次の辞書です。
・Urban Dictionary: U.S. Marine Corps- to relieve oneself of urine and or feces in a designated location such as the head (restroom): Good morning sir. This recruit requests to permission to speak to the Drill Instructor. This recruit would like to request permission to make a head call.
-Frickin darn head call?! Now?! Are you getting ready to soil yourself?

"head" は俗語でトイレの意味で、これは普通の辞書にも掲載されていました。
・Oxford English Dictionary: Nautical: A toilet or bathroom on a boat or ship.: they were cleaning out the heads
・Collins Dictionary: Nautical a toilet, or lavatory
・Cambridge English Dictionary: (nautical) The toilet of a ship.: I've got to go to the head.

なるほど、それで "drill instructor" が "I hear voices in my head" と言うのが可笑しい落ちになるのですね。