So I say fuckit, I'll go out and buy a test set and a 66 punch block tool and check out the demarc myself. That's $200 I figured I'd eventually spend, I just needed an excuse, which I now have. I get back from Home Depot and check the demarc, which requires contortion to get to.
It's a not a modern demarc--not the modular plug & jack junction designed for Joe Sixpack to plug a phone into, to make it real easy for him to determine if it's his problem he has to pay to fix, or if it's the LEC's problem. (More likely, so the LEC can tell him "We told you so" when he disputes the charge when their tech determines it's an inside problem.) It's a 66-terminal block. And it's behind a padlocked door. I don't feel like waiting around all day for the complex's maintenance droid to show up to unlock it for me, so I have to climb in via the (unlocked) cabinet door for the electric meters next to it. I clear the cobwebs and climb inside, and attach my test set. At which time, the phone works again, and I gotta change my pants 'cause everything in there was dirty.