I have successfully hardwired the Earthlink internet digital phone into the existing phone lines of the house. Hot Dog!
I can't find the link to the article that I used, but this one has most of the information you need.
THe most important thing to remember is to disconnect the phone companies incoming lines at the box outside of the house (single family homes only - i'm not responsible for you mucking up your apartments complex's phone system). Disconnecting these lines protects your VoIP box from any random ringv-voltage that the phone company might decide to send into your home.
Hurray for me.
EDIT: here is the link to the document that helped so much.