Station Protector at the entrance point. Some common examples are shown below as well as an open NID showing the customer access side with a single Entrance Bridge Network(EBN) module installed.
Since a NID is where the phone company's network stops and the home owner's network begins, it's a great place to isolate problems. The EBN (there may be more than one EBN depending on how many phone lines your house has) inside the NID will have a test jack where you can directly connect a phone or modem to the TelCo network and eliminate your inside wiring.
For more detailed instructions about testing your modem or circuit please reference the NID Test FAQ.
Andy Houtz DSL