If you're savy, you can disconnect your house from the outside phone lines (so the lines inside your house are "dead") and then plug a simple phone line into your adapter to your wall jack, then all phones in your house are connected to the VoIP service.
There are two potential problems with this:
1) You must make sure you disconnect the outside phone service - things won't work right and you could even ruin your VoIP hardware.
2) Some VoIP adapters can only supply enough current to power one or two phones. You'll want to make sure you don't overload it by plugging too many phones into the jacks throughout the house.