How could a landline be re-routed?

Indeed, and serviced by the same central office.

In most cases there is. Or at least from your phone to the nearest SLC.

Um, what? Of course it's /supposed to/ be a single pair for the line. At least in most set ups. What are you talking about here?

Actually the pair from your home to the CO is a collection of pairs connected to one another. Pair from house to street cable connected at pole or underground pedestal. The street cable is connected to a larger cable that runs along a major street and so on to create a single pair back to the CO. Of course, that's the old fashoned way. Now you are likely to be connected to a fibre concentrator well before hitting the CO.

What I mean is this: Let's say that I live at 123 Maple St. and you live at 234 Maple St. There is a 100 pair cable serving the street. I have a phone number that comes to me on pair 57 of that cable but I have it disconnected because I'm happy using just my cellphone now. You order a new line from the telco. They assign your number to what is now a vacant pair on that cable, pair 57. The installer connects your drop cable to pair 57 at the pedestal and voila, you have phone service. If I leave my old dead phone plugged in and the installer doesn't stop at my place to disconnect my drop from pair 57 then I too have your new phone number working at my place. This happens far more often than the telcos will admit. It is a real problem in office buildings. I can't tell you how often I've found other companies lines appearing in a suite that isn't theirs.

Bruce is correct here. I've seen it numerous times in commercial premises. There will be one or more 200-pair bundles, all punched down to 66-blocks in the phone room. Checking with a telephone butt set you will find lines connected to other business in nearby buildings.

Yes, but there'd still be ONE pair for the POTS line leading from your house to the nearest piece of equipment. Once upon a time that would have been the central office. Yes, you'd be pulling one pair up to the pole (possible inside a multi-pair overhead line). From there it'd probably get spliced into a multi-pair cable. Possibly again when that cable lead underground to a larger bundle going back to the central office. Binding post charts detail all of this. There are all sorts of ways the pairs can get screwed up.

Ah, then you're talking about incompetence on the part of the worker (who may or may not be an 'employee' of the telco). I've certainly seen plenty of situations where an office building had distribution points throughout the building that had pairs from other businesses running through it. Not since the days of Centrex handselt have I seen the wrong lines ringing at handsets. Now most lines run into PBX equipment of one sort of another. It'd be odd to have both the pairs setup wrong AND the PBX configured to use the wrong trunks. Not impossible, but a lot less likely than in the past.

The message posted wasn't really clear on what was actually happening, or even when.

If it were a small business they may just have a key system instead of a PBX, the difference being that the CO lines for a key system appear on the phones.

I was living within 10 miles of where i worked and my understanding is that an off-premise extension includes the home phone into the broader PBX business line, so extensions could tap into the home line off of that

Sorry, I don't follow what you're explaning.

You're using terms that don't quite match up with telco terminology.

Any number of things are possible depending on how you have service from your telco. It's trivial to do call forwarding to re-route an extension at work out to another phone number. But unless you're using the ILEC for the PBX services it's not as easy (nigh on impossible) to have that remote number act as an outgoing extension.

so they heard you coming and ran off?