With that said, the best way to show this is by a series of pictures... please, don't smack me... a graphics artist I am NOT, as the following series of images will clearly prove. :P Remember, this applies only to 'twisted pair copper lines', and not the newer forms of fiber optic lines that some areas have been converted to. The best way to start, is at the beginning, yes?
When a telephone exchange is designed, the phone company will calculate out how many customers it will need to service, and then they will install several sets of feeder lines to various "groups" of locations, and then split the master feeder into chunks of smaller feeders. For example, a master feeder might be a 500 pair circuit and sub-feeders might be 100 pair lines, which could further branch out into 25 or 50 pair circuit sections.
THE most important thing to remember is that each sub feed and smaller feeds, or any 'drops' made along the way in the system, are all called "taps". And each 'tap' is a "parallel" connection. Think of it as a set of Christmas tree lights, when the light in the middle goes out but all of the other lights remain on.
It starts like this:
The middle break down would look something like this. Simple concepts being what they are, yes?
The "end"... of the beginning. Sorry to say it's not the end end. You will see why shortly.
Multiple End Points
At this point, you need to know there are some rather technical things involved. Phone lines are low power devices, that operate on a DC voltage versus your AC type house power. You can usually hold both of the phone wires in your hands and not get zapped. (Oh yeah, compulsory YOU ARE STUPID warning here. Don't do that with any part of a wet body... and *pray* the phone doesn't ring while you are holding the wires... because you will get zapped!) With the line being low voltage, the distance it can travel is limited. The more copper you put in place, the more resistance the distance of wire adds up to, the less "power" there is at the end of the run.
End point #1. So now you understand that the farther you are away from the exchange, the lower the quality of service might be. Normal voice phone usage isn't generally affected by this BUT services like ADSL and TV over the phone line ARE affected.
Remember, this is just a simple explanation.. Another critical technical matter is called "capacitance". This comes from having 2 (or more) items running close to each other. In this case, the items are wires. Now capacitors can do several things. Depending on the value of the capacitance, and the frequency of the transmission on the wire (voice band, Internet band, TV station band) it IS possible for any capacitance value in the lines to leak or bleed or maybe even short out one type of signal. Needless to say this is a HUGE problem for the phone companies, since the normal twisted pair copper wire system was NEVER designed to handle more than just the normal voice band. And each "leg" of the wire run, no matter if the other end of the wire is connected to anything or not... will change the capacitance value of the line. Where do you get more "legs" attached? Ah yes... those nasty drops that people have added into their homes (running a small business in their house, maybe need 5 phone lines) or a new strip mall that might need 50 lines.
You see, if you need say 5 lines well, the phone company won't run in 3 separate lines of 2 pairs each, they will probably run in a 25 pair cable. And in this case, when they run the new cable into "the house", all 25 pairs WILL be hooked up at the connector block. And in the real world, this happens over, and over, and over again.
So in order to deal with a capacitance issue, the phone company MUST send someone out, and check every junction point. IF "your" phone line happens to be 'tapped' into something else then they have to remove that tap. Kinda like unplugging that extension cord. Once it's no longer plugged in, there is no power at the end, and nothing to pick up or cause any electrical interference.
The above paragraph also describes End Point # 2, which is *exactly* how someone can potentially access your phone line. Because in the 25 pair case above, one of the "non used" sets could have been attached to the same connection your line is on. All someone in the house has to do is stick a phone on each of the other pairs, and see if they get a dial tone.
With all of this known, we are left with End Point # 3, which deals with static, hum, cross talk, line buzz or other similar situations. The copper wires are small, with a thin plastic surrounding the wire. The cables are strung on poles outside subject to weather. When it gets really windy, the cable can stretch, and cause lines to break. The plastic rubs on things and can get worn out to the point where it causes a short making the line dead. The cables are protected with ground shield lines, which can cause ground faults and ground loops (aka hum on the line). The last person servicing the line may not have sealed the connector box properly allowing moisture to get in, which causes hum and capacitance issues. Really, the list is endless.
So depending on your problem, your distance from the exchange, how many tap points there are involved... the phone company might be trying to pull the wool over your eyes. Or maybe they just don't want to put in the effort and expense.
Always remember when you deal with anything other than VOICE over your phone line, the distance to the exchange will control your quality. You may find that surfing the net slows down when the phone rings or is being used. Maybe your television channels go a little flaky under the same conditions.
This is of course, all just food for thought... a *very* simple explanation. Real life says there are a lot more taps and drops than this picture paints.