"But what about using your VOIP phone throughout the house so that you are not tethered to the room that has the ATA box and the high-speed Internet modem"
We are creatures of habit, and once we get used to doing things a certain way, it really is difficult to change.
For some people, this also applies to implementing home VOIP service in their home. Unlike traditional phones, where your home may already be wired with a phone jack in the wall in each and every room, your home is probably not wired in the same way for VOIP. But you can still implement VOIP throughout your home, and there are multiple ways to accomplish this.
Let's look at the component parts of your VOIP system. Once you decide which VOIP provider to use, you will get a VOIP adapter box, also known as an ATA box. This box is about the size of a deck of cards and is the "smarts" behind the VOIP. One cable from this ATA box plugs into a spare jack on your high-speed Internet router, and your standard RJ-11 jack from your regular phone plugs into the other end of the ATA box.
Ok, some of you are saying you have a problem already, since your high-speed Internet connection modem only has ONE jack on it, and that is where you plug your computer in so you can get online. Yes, this is a problem but not a huge one. You are going to need to go to Best Buy or Radio Shack or some similar place and get a HUB. The easiest way to think of this is like being an extension cord for your high-speed Internet modem. You plug the hub into an electrical outlet, and then you run a cable from the hub to the jack on your high-speed Internet modem. Now you plug your computer into one of the jacks on the hub (you will probably have about 3 or 4 of them), and also plug the cable from your VOIP ATA box into another jack on the hub. Before you leave Best Buy or Radio Shack, make sure you have the cables with RJ45 plugs on each end - one of these goes from the hub to your modem and another one goes from your VOIP ATA box to the hub.
Now with your VOIP ATA box connected to the hub or directly into your high-speed Internet modem if you had a spare jack on it, you just connect your standard traditional phone into the RJ-11 jack on the ATA box and you are good to go.
But what about using your VOIP phone throughout the house so that you are not tethered to the room that has the ATA box and the high-speed Internet modem? You have multiple options here. You can use a cordless phone where you would plug the base unit into the VOIP ATA box, and then you are good to go. I would recommend a cordless phone that is in the 5.8 Ghz range to provide your best coverage.
If you want to have multiple phone handsets around the house, I would recommend getting one of those multi-unit cordless phone setups. Some of those can accommodate up to 8 cordless phones with the same base unit.
There are places online that will explain how to wire your VOIP ATA box into the outside wiring of your home phone system so that the RJ-11 jacks you already have in every room can still be used. While this is possible, it is not recommended. There are multiple stories of people who have blown out their ATA box and/or the entire phone wiring of their home because they were not familiar enough with the electrical load requirements of implementing a system like that. Unless you are an electrician or telephone company repair person, this approach is likely to cause more problems than it solves.