I think I see what you're trying to figure out.

You need to have the phone and fax plugged into the Vonage Phone Adapter Router (my Linksys Phone Adapter Router has 2 phone ports). So... there are two ways you can have multiple phones throughout the house on Vonage:

1) Connect a cordless phone's base unit to the Vonage Phone Adapter Router and put satellite units in other rooms where you want extensions. Don't get a 2.4GHz cordless phone because that can interfere with your wireless router. Stick with either 900Mhz or 5.1GHZ (or 5.4GHZ... can't remember the exact spectrum... but it's 5 point something!). The problem with this method is the fax machine as it will need to be plugged into the Vonage Phone Adapter Router.

2) If you are ditching your POTS phone (Plain Old Telephone Service... aka landlines) you can connect your house's phone ports in the wall to your Vonage Phone Adapter Router. Then you can simply plug any phone into the phone ports in the wall. You'll need to make sure that the port where you plug in the fax machine is connected to the correct phone port on the Vonage Phone Adapter.

3) I know I said there were "two" things you could do... just thought of a 3rd. Some people keep their landline service and modify the account to the absolute minimum service plan you can get and use that for their fax machine (a double adapter enables you to have a regular phone plugged-in, too, so that if power or internet fails you still have a phone).

I got an Airport Express before I got Vonage... and I stream music from my computer to my stereo... so I need to have both my Linksys Phone Adapter Router and the Airport Express. If I didn't care about streaming music I could use just the Linksys Wireless Phone Adapter Router which broadcasts on 802.11g... with is compatible with Macs that have Airport Cards and Airport Extreme Cards.

It might seem a hassle to try to figure out how to make Vonage work for you, but in Boston I'm finding that my minimum savings per month is $60... that's a lot!

I would suggest if you wish to re-wire your home's phone ports to work with Vonage that you visit Vonage Forums. Don't be scared by the numerous posts about problems... it's a very small minority of Vonage users who have very vivid imaginations.