

I remember coming across the thing you’re describing years ago while digging through my dad’s collection of miscellaneous cables, adapters, etc. back in the 90s or early 2000s. It wasn’t quite so low-profile, it definitely stuck out from whatever you plugged it into maybe about a quarter to half inch or so, but otherwise it was a 3.5mm jack with a plastic cap on the other with no wires or holes or anything that muted whatever you plugged it into.
The shade of beige the plastic was on that particular example makes me suspect it was a relic of the 80s. I do feel like I remember seeing them for sale somewhere at a later time, but I couldn’t begin to tell you where.
A little googling turned up this eBay listing
Based off of that and a little more googling I think the term you’re looking for might be a shorting and/or blanking plug or or cap or dummy/dummy plug
Without too much effort I was able to find “shorting caps” for RCA jacks, various coaxial connectors, and banana plugs, but had no luck finding any more for 3.5mm
It’s absolutely not a job for everyone, but assuming you’re in the US, damn-near every 911 dispatch center in the country is always short-staffed and hiring, and usually only require a high school diploma or GED.
Since you have a computer background, I think it’s safe to assume that you can type at a halfway decent WPM, that’s a pretty big chunk of our aptitude test that a lot of people fail on.
A lot about this job varies from one jurisdiction to another, but in general pay is livable but not amazing and the hours are usually weird, but the benefits and job security are pretty solid.
Background checks, drug testing, etc. are of course usually part of the hiring process, and again it’s just not a job everyone is cut out for.