To further clarify using your existing house wiring with a VoIP  phone:

Unplugging the connection at the NID works great if your using
cable for your Broadband service. If you're on DSL, you need
the Telco wiring to get DSL signalinto the house and to the DSL
Adapter. You can still use your existing house wiring for VoIP,
but you will probably need to rewire your phone jacks to use the
other 2 wires (of the 4) to distribute your VoIP phone in the
house. This isn't hard, but a cordless phones would be much easier!