Generally, Vonage has its own phone jack on the adapter that you will use for your phone. It is not intended to be used with your house phone wiring.
If, however, you wish to use your home phone wiring (which they do not recommend), you must disconnect your home service from the jack outside of your home and then connect the live vonage line into any phone jack.
Thats what I DID. All you have to do is diconnect the outside wire source and then plug vonage into any phone wall jack and all the phones in the house will work.
I think they don't recommend it for liability reasons. You just know somebody is going to hook it to their house wiring without unplugging the incoming lines and their equipment will blow. Having this disclaimer in there saves Vonage from having to replace a bunch of broken equipment.