To begin: A single telephone voice line requires one pair of wires. This makes sense if you think of it electrically -- there have to be at least two wires to make a complete circuit.
The two wires that form a single pair are referred to as ring and tip. The names come from the early days of the phone system, when operators at the CO (Central Office) physically unplugged, moved, and reinserted large microphone-esque plugs in a giant switchboard, to connect individual calls. Those connectors had a ring part and a tip part, just like a microphone jack.
"Just lost Peoria!"
Polarity of the two wires used to matter, but with modern phones, it doesn't. However it is best to stick to the standard, and most installers do. The ring wire, therefore, is almost always red -- remember the two R's. Green is tip.
<— Red and green!
With houses and other small, residential buildings, the phone company supplies wires from their pole or underground lines, and terminates them in a small junction box or demarc (as in demarcation, and pronounced "DEE-mark" by the guys whose pants hang too low) or network interface device or NID (pronounced "nid" by the aforementioned low-hanging-pants guys), attached to the outside of the house. I will henceforth refer to it as "the box." The box is typically plastic, about a foot square or a little less, with a weatherproof gasket and a screw holding the cover tightly shut.
The box: —>
aka NID aka demarc aka whatever.
Modern boxes are stamped with the words "customer access" and an arrow pointing to a normal screw; there is a sub-cover, usually held shut with a recessed hex bolt, which is supposedly for "telco access only." The customer "side" of the box typically contains the wiring posts for each of the house's internal phone lines, and a customer test jack which is simply a normal, wall-style jack for a normal phone cord. The idea here is that, if a problem occurs, the customer can open the simplified side of the box and try the line with a standard phone; if the line works at the box, but not inside the house, then there is a problem inside the house, which is the customer's problem. If the line is dead at the box, then it's the phone company's problem to fix. They only guarantee their service up to the box.
<— A fairly modern box.
Sometimes, in older boxes, the customer test jack has a short phone wire tail and connector already plugged in. In this case, keep in mind that when you unplug that little tail and plug in your own phone to test, your house's wiring is now physically disconnected from the box. In other words, the little tail bridges the internal wiring to the company's wires. This may make more sense after you read below.
A rather old box. —>
Most residences have a standard two-pair wire coming from the street to the box. Most houses only ever use one of the pairs (i.e. most people only have one phone line.) The second pair is physically capable of being used as another phone line with another number/account associated with it, but it is by default not activated. This second pair is usually black and yellow, and is sometimes known as the "Halloween pair." Here, yellow is the ring side, and black is the tip; I know of no good mnemonic to remember this, except the poisonous snake rhyme, which starts out "red and yellow kill a fellow...". This is how I associate red and yellow as both being the ring side. Again, these days, polarity is not crucial.
"Bruce" writes: "I like your snake mnemonic red and yellow kill a fellow, especially since ring carrries the negative voltage and tip is ground. I call the second pair the 'bumblebee pair.' Bees attack with tip of black." Cool!
Backing up a little, to the customer side of the box: The wiring posts. In some boxes, they are exposed when you open the main customer access cover, but in other boxes they each have their own mini-cover. The phone company's two pairs run to the back of the box, and each pair is in turn connected to one set (pair) of screw-terminal posts. You can't see those wires from the customer side. The posts are labeled and color-coded red/green for polarity, and it is important to note that even the posts for the second pair from the street are red and green. Those are the meaningful colors in telco stuff, and the other wires are only black and yellow to distinguish them from the base pair. It might've been less confusing to go with red and green-striped insulation for the 2nd pair, but they didn't do that thirty years ago, so they still don't.
<— Nice detailed view of a demarc.
Most boxes have 3 or 4 pairs of wiring posts, even though most of the time there are only two wire pairs coming from the street. I supposed the logic here is that it's trivially easy and inexpensive to run another set of wires from the street, but it's more involved and costly to swap out a whole box for that relatively-rare person who needs more than two lines. So they just provide the extra posts even though most people never get more than two phone lines.
The wires that you do see going to the posts, are the wires that go inside your house. The posts are where your house's wiring meets the company's line from the street. In the same way that the phone company runs two pairs of wire and usually only uses one, your internal house wiring consists of two pairs (four wires total) to EACH wall jack. You typically only use the base pair, and in fact, most phones you buy today come with a phone cord that only has two wires in it. They are the center two wires out of four possible (if you peer carefully through the clear connector at the end you will see four grooves for four possible wires). Business-style two-line phones have cords with four wires, and if you have two phone lines going to one jack, the 2nd line uses the Halloween pair (which is the two outer wires). OR you can get a "2-line splitter" adapter at Radio Shack which splits the one jack with two lines, into two jacks each with one line. Confused yet? :) Again, it's about redundancy...most people have just one line, but for those who need a second line, this enables it to be used without making a new hole in the wall and pulling a new wire.
A two-line splitter thingy. —>
<— Another one.
Do NOT confuse the two-line splitter adapter mentioned above, with the more common, identical-looking duplex adapter. The two-line splitter takes EACH pair and sends it to the center two wires of each of two single jacks; a duplex adapter simply duplicates the center pair in two jacks.
^ This explains everything.
In a perfect world, for each jack in your house, there would be one bundle of four wires (of which you usually only use two), going to the junction box and connecting to the wiring posts. Unfortunately, this is not how it always works. It is common, instead, for some OR all of the phone jacks in a house to be "daisy-chained" together, with only one set of wires coming out and connecting to the actual box. Furthermore, in some houses where this is the case, the Halloween pairs are left disconnected. Then, in the event that you DO want to use your second line, it seems dead after the box. This is horrible to troubleshoot if you have a problem. You have to unscrew each phone jack wallplate, check/connect all four wires to their proper jack posts (they are usually labelled, thankfully), and then test and hope that all of the wires between the jacks are intact.
Regardless of whether or not your home wiring involves the daisy-chain nightmare described above, damage can occur to the base pair -- a staple or a nail can sever a wire or short the pair together, and you either have to expose/replace the whole wire, OR, if you're not using the Halloween pair for a Line 2, you can change over to that pair. In other words, you abandon the damaged base pair entirely, and use the 2nd pair as an in-place replacement. It can confuse you and/or the phone guys later, and you're screwed if you need to add a second line to that set of wires, but, in a pinch, it works. You have to rewire the jack as well -- so that the Halloween pair connect to the middle two wire positions in the jack. Only the aforementioned special two-line phones can make use of the outer pair, normal phones will only work with the inner pair.
A phone jack, wires exposed.
Tools: You can do almost anything related to phone wiring with relatively normal tools (screwdrivers, wire strippers or scissors, a multimeter with a continuity or resistance function). But there are some specialty tools that you can get, so that your pants slide down a bit. One of these is a lineman's handset or buttset (I swear I am not making that up.)
^ A buttset.
A lineman's handset is really nothing more than a glorified telephone. It has extra buttons and bells and whistles which do not concern us; the main feature is, the alligator clips. They are the special insulation-piercing kind, though you usually don't need that option. Its real value lies in the fact that you can connect directly to a set of wiring posts, using the non-piercing ends of the clips, OR you can clip onto the ends of a bare wire.
A nice one like the one pictured, can easily cost you $80 or more new. I found one at a surplus electronics company for about $20, working except for having had its clips clipped. So I also ordered a pair of the clips, got some wire, and repaired the thing. However, you can make one out of a cheap phone, provided it's the right kind. This is somewhat hard to describe, but, the phone needs to be the kind with all of the circuitry, INCLUDING the hang-up button or hook, in the handset. This can actually be difficult to find -- most electronics places have phones that are too "nice" for this purpose, i.e. they are too complicated and cannot function without the base AND handset, and that just won't do. Try a chain pharmacy -- many have selections of small, cheap appliances, and here you are most likely to find the cheapest, shittiest telephone on Earth, which is just what you want. The kind that doesn't have a base, but rather that you "hang up" by setting down on a flat surface. It should not cost more than $10.
<— A cheapo phone suitable for transformation into a cheapo buttset.
A "slimline"-style phone will also work, provided that, again, the hanger-upper is in the handset. Note this carefully in the picture. Sometimes there is a hangup button on both the base and the handset -- that's okay too -- but many have it ONLY in the base, and those aren't suitable for conversion, at least I haven't been able to figure out how.
Once you have the phone, the rest is fairly simple -- cut the wire, discard the base, and crimp or solder two alligator clips on the ends of the two center wires (regular clips from Radio Shack will do, you don't need the insulation-piercing kind, but if you can get them with rubber sleeves, do so...when they're clipped to the ends of a wire, they tend to swing around and bump into each other and cause shorts.) The wires in the phone cord will probably be red and green, but not necessarily. The tricky thing here is that they are typically tiny, delicate wires, and if you don't provide some kind of support or strain relief, they will quickly fatigue and break where they are attached to the alligator clips. So what I usually do is carefully slice the outer insulation of the wire right down the middle, strip (sometimes with fire) the wire, and run it and its half of the insulation into the collar of the alligator clip together, then crimp down on both (making sure that the actual wire makes contact with the alligator clip's metal collar.) This way, the insulation acts as a splint for the thin wire inside.
Now, for the really cool tool, which is worth whatever it costs:
^ The Almighty Tone Generator and Probe Kit.
You take the small square part with the wires dangling off of it, put a 9-volt battery in it, plug its modular connector into your wall jack, and turn on its toggle switch. Then you go to the box and poke the probe near the bundle of wires, and the probe will make an audible warbling beep that gets louder as you get closer to the right wire. It's pure magic. Actually it's induction and is quite simple...nah, it's magic. When you have a meaningless and frightening bundle of wires to confront, and you need to find the other end of one, there is simply no substitute for this tool. The alligator clips on the tone generator allow you to use it with bare wire as well.
That's all for now...