Its totally up to the building owner how they deal with it. In most of the bigger apartment buildings I've done work in, the telco will be in (usually, sometimes forgotten to be) locked closets along with the cable TV plant. Sometimes with some pretty beefy locks (ie. Medeco, that I had to leave behind my drivers license to get the key from the building people).
Smaller/older buildings may just have it somewhere in the basement, or an outside NI box hanging out somewhere, maybe locked, probably not.
Its more of a matter of people don't usually mess with it because they have no idea what they are looking at other than a bunch of wires behind a door/inside a can/behind that panel, not even realizing they could do a bunch of messing up people by messing with all the wires. Plus the ones that could really mess with it and have punch down tools, test sets, etc. could do the same sort of problem from many places, not just one specific NI box somewhere.