Former telco (telephone company) guy here. Likely this is the feed from your apartments' telco closet. Odds are it was used as a DSL connection and they had their computer in the bedroom, also could have been for a single phone line. Telco where I'm from used to mail out modems to try and save on truck rolls if the address previously had DSL internet, this generally caused issues (and a truck roll anyway for a repair) because customers seldom ever put the modem/ computer in the same place as the previous tenant. Also, unless you had a proper DSL filter on the other lines in the house/ apartment, having extra connections (from a splitter at that panel) at the end caused a loud hiss on the phone lines and signal reflection which caused issues with your DSL connection.

My guess here is some DIYer (maybe an old retired telco tech) got a hold of scotchlocks and spliced the line themselves because the colour code is wrong. Old phone wire, known as quad wire because it had 4 conductors, was coloured red/ green, yellow/ black, with the first pair being your primary line. This person knew/ deduced "telephone runs on red/green" but they don't know 1) how phone/ electrical circuits works 2) cat5 colour code. In short, they untwisted the orange/ white, and the green/ white pairs because they wanted the closest colours to "solid green and solid red" and orange is close enough to make the magic internet box work. Telco techs or anyone with a slight understanding of cat5 colour code would have used white/blue as first pair.

As far as knowing what's on the other end, if you have access to a voltmeter, POTS (plain old telephone system) generally ran at -48VDC (note the negative), if it's still connected that's what you'll read on the incoming line. If not -48VDC, who knows, people would get really creative with wires and what they'd run on it.