Most houses these days are pre-wired internally for two phone lines. In truth, I have never seen it any other way. However, there are always exceptions to every rule; so I'll stick with "most".
Before I belabor the details, it was unclear whether your house is brand new, or just new to you. If your home is brand new, then inform the builder. All of the phone jacks should work, so you can add this to your punch-list of things they need to correct. If your home is an older one that you just moved into (like mine) read on. I'll try to be brief, but some explanation is necessary to help you understand what you are looking for.
Most telephone components have standardized on a 4-wire system. Modern jacks that use the DB25 plug (that's the little clear plug on the phone cord you stick into the jack) also utilize these four wires. When you remove the cover of one of the jacks, you will see four screws, and attached to each will be a short wire leading to the part of the jack you stick the plug into. The colors will be red, green, black, and yellow. You will also find one or more cables coming out of the wall if a wall-mounted jack, or somewhere else if a surface mount. I have seen cables run in houses that contained other color combinations, even more than just four wires, so you may want to write down which colors are connected to each screw on the jack, so you will know which internal wire corresponds to the jack's wire. Hopefully, though, red, green, black, and yellow will be all you see.
Now that I have explained the wire colors, here's how they work. The red/green wire pair should represent the primary line, and the black/yellow the alternate, or line 2. I know someone mentioned that each of the 4 wires can correspond to its own phone line. I do not believe it is true for 2 reasons; 1) like any other electric appliance, there has to be a return loop for the electric current, and 2) since all off-the-shelf phones one can buy are using 2 wires, they would be incompatible in a single-wire setup. Now it is true that modern homes often have a main phone junction box, where up to 4 phone lines can be set up without additional cables from the telephone pole, but for lines 3 and 4 additional wire would have to be run into the house. Of course, technology changes daily.
In the old days a phone line needed all four wires in order to operate. Gradually, technology improved, or at least changed, and today a single phone line only needs two wires. Since most houses have at least four wires, adding a second phone line often is pretty easy. The most work is making sure that the black/yellow wire set are connected everywhere. This is because houses, built before two phone lines were common, often never got the black/yellow wires hooked up. The ends just dangle behind the jack cover. Each phone jack in your house is wired in series from one to the next. If you wanted to add a new jack, you would simply need to run some phone wire from the nearest existing jack to the location of your new jack, and match up the colors. Popping off the baseboards makes a nice place to hide the wire. Of course, people do simply staple/tape the wire along the wall instead of removing the baseboards. Itís your call. As far as the telephone components you need to buy, they are widely available.
All this may sound confusing, but essentially, it's a simple concept. If you have only red, green, black, and yellow, all the wires should be connected together, color matching color. If other colors exists inside the walls, for instance, a white with green stripe from the wall is matched to a red on the jack, then that pattern would need to be matched at each jack, and so on for the other wires. Any difference, or unconnected wire may mean phone jacks downstream will not work. Therein lies the problem with your non-working phone jacks, I believe.
I would start with the jack that was used to plug the computer into. Most computers default to use the primary wires to dial out on, and modems usually do not allow for switching to the alternate wires. This means that when you plug a computer to into a normally-wired jack, it will dial on the red/green wire set just like all your phones. If the previous owner had two lines, he no-doubt wanted the computer to use line-2 so, for example, he can be on the computer and, at the same time, order a pizza over the phone. A standard solution to this is to reverse the wires on the jack used for the computer. To do this, the previous owner would have had to take off the jack plate in question, loosen the screws, and switch the red/green jack wires with the black/yellow ones. I'm referring to the wires that are part of the jack plate, not the ones in the wall. In this way the computer will use line-2, and the phones, line-1 That is exactly what I did in my home. I canít remember off the top of my head, but I believe red matches to black, and green to yellow. Itís not important since you donít have a second line, and just want to make the jacks work properly.
The problem is, the previous owner may have wired other jacks to use line-2 as well, for a fax, extra phone, or whatever. Since you do not have a second line, any jacks so wired would now not work. As another scenario, the previous owner may have inadvertently disconnected the wires in the wall, and if he did not use the other jacks down the line, would not have realized that they no longer worked.
What he was trying to accomplish may not matter so long as you can correct it. I would suggest comparing the wiring pattern of the computer jack with the jack of the phone that works. Iím reasonably sure that the wires on the computer jack are reversed. If so, match up the colors and re-connect. You should be moving the little wires that are part of the jack, not the ones coming out of the wall, but most importantly, when you are done all the same-colored wires should end up on the same screw. Hopefully this solves your problem. You will loose the feature of having a computer dial out on line 2, but right now you donít have a second line, and the goal is to get all the jacks working on line-1.
If you still canít get the other jacks to work, you will need to check all the other jacks for inconsistencies. If, after checking all the jacks and getting the wires to match up, you still can't get anything to work, you may have a broken wire inside a wall or somewhere. As a last resort you can always call in the phone company.
A second way to test the wires without moving wires around would be to borrow someoneís two-line phone. A two-line phone simply has two circuits for the red/green and black/yellow lines. By plugging it in at each jack, you may find that with some jacks that the phone only works when the line-1 button is depressed, while at other jacks, only the line-2 button works. This simple test would prove that somewhere at one of the jacks the wires are reversed, and my bets are still that the problem lies with the one used for the computer. As a third option, if possible see if the previous owner can be contacted for information. At some point, though, you may still need to move some wires around to correct the problem.
Sorry I was so long-winded. I could not figure out a shorter way. If Iíve confused you, show this note to someone who has both a computer and 2 phone lines. Chances are they know exactly what Iím trying to say.