Thanks. Here is the link from the Vonage website:
Sorry to have bothered people here because they make it perfectly clear. For the record, here is what they say:
One way to use Vonage on multiple phones is to modify the existing telephone wiring in your home to distribute the Vonage service to all of your phone jacks. Then you can plug a regular telephone into any jack and make a call.
This option works best if you own your own single-family home. If you live in an apartment or a multiple-family dwelling, chances are your landlord and neighbors won't want you to mess with your building's telephone lines. It also helps if you are handy around the house and have a basic understanding of telephone wiring. It's not very difficult to modify your home phone wiring, but because you're dealing with lines that carry voltage, there's always a risk of causing a fire or damage to your phone lines and equipment. If you're not comfortable doing the work yourself, you should hire a professional electrician or telephone technician to do the job instead.
It's important to note that by modifying your telephone wiring to distribute Vonage throughout your home, you'll be totally disconnecting yourself from the phone company. But the process is completely reversible. So if you sell your house in the future, for example, you can restore your old phone configuration with minimal difficulty.
STEP ONE - ISOLATE YOUR INSIDE WIRING
To re-wire your home for Vonage, you first need to isolate your inside phone wiring from the lines that come into your house from the phone company. This is a step you shouldn't skip, even if you think your phone line is already dead. If you don't isolate your inside wiring, and the phone company decides to send voltage across the line you thought was dead, it could damage the telephone equipment inside your house or worse, cause a fire.
To begin, find the box on the outside of your house where the telephone lines come into your house from the street. This is called the Network Interface Unit (NIU). It's the legal demarcation point where the outside wiring from the street (owned by the telephone company) meets the wiring inside your house (owned by you). When you open the box, which is usually locked or fastened with a screw, you will have access to the side containing the wires going into your home, but not the side with the lines coming from the street. You'll also see a ground wire coming out of the phone company's side of the box. This wire protects you against lightning strikes, so make sure you never disconnect it.
Once you've opened your side of the NIU, you'll see one or more sets of screw terminals inside. Each will have a short piece of telephone wire coming out of it with a phone connector on the end plugged into a corresponding jack. If there's only one line coming into your house, you'll most likely have only one set of screw terminals. To disconnect from the phone company, simply unplug each of the short telephone wires from its corresponding jack.
Next, you need to make it obvious to others that you've unplugged the wires on purpose and they shouldn't undo your modifications without risking damage to your inside equipment. Start by wrapping the end of each of the telephone wires you just unplugged with electrical tape so it can't be plugged back in without unwrapping the tape. Then, clearly label the inside of the box with a message that says something like: "Do not reconnect! May cause damage to inside equipment!" A sign written or printed in waterproof ink and taped inside the box works well. No matter how you choose to label the box, be sure it is obvious, clear, and easy to read.
Once you've clearly labeled the inside of the NIU, close and refasten the box. Then, just to be safe, label the outside of the box as well. To be extra safe, you can also wrap a cord or nylon tie-wrap around the box so it can't be opened without cutting it. Remember, to avoid damage, you want to make it as inconvenient as possible for someone to change what you've done without your knowledge.
STEP TWO - CONFIRM THE LINE IS DISCONNECTED
After you've isolated your wiring from the phone company's, it's important to confirm the line is disconnected before installing Vonage.
Go back into your house and pick up a phone plugged into a jack that previously worked. You should hear absolutely nothing; the line should be totally dead. If the line's not dead, go back and check your work. If your work looks correct and the line's still not dead, it means that voltage is somehow still being carried on the line and it's not safe for you to proceed any further. Consult a professional electrician or telephone technician for help.
STEP THREE - CONNECT YOUR PHONE ADAPTER
If you've successfully isolated your wiring and you've confirmed the line is dead, the hard part's over. It's time to connect to Vonage!
Simply plug your DSL/cable modem into the Vonage phone adapter. Then plug your phone adapter into any telephone jack using a standard telephone cord. Finally, plug regular phones into the other jacks in your house. Telephone jacks are wired in parallel, so when you plug your phone adapter into any working jack, it will spread the signal to the other jacks in your home.
Like any telephone line, there is a limit to the number of phones you can connect to a single Vonage line. If too many phones are connected, the signal will fade, and not all of the phones will ring when a call comes in. Therefore, we recommend you only connect five phones maximum to a single Vonage line.
Congratulations! Your home is now wired with Vonage!