End of the Local Loop

This box is called the Telephone Network Interface by the telephone company.  Any problems on the customer side of this interface are the customer's, so it is a good idea to test for dialtone here before you make a trouble call.

This connection box is just inside the house, on the other side of the wall from the Telephone Network Interface.  Note the two wire pairs -- red/green and black/yellow, each for a separate telephone line.  If you have a voltmeter available, you can test the voltage for each pair.  If there is only one line, it will be on the red/green pair, and you can expect it to read less than -48 volts DC.  (It will most probably not be the value you calculated in our earlier exercise, since there are probably repeaters and coils in the line between your house and the central office.)   If there is only one active line, the black/yellow pair will have no voltage.

Typical telephone wiring.  Note that even though the wire is referred to as being made of "twisted pairs," neither the flat wire (top) or round wire (middle) is divided into pairs or has much of a twist.  Data communication wire, such as Cat 5, Cat 5e, and Cat 6, has very specific requirements of twist and pairing.  The gray wire at the bottom is terminated with an RJ-11 (Registered Jack) modular jack.  This is similar to, but does not take as many conductors as, the RJ-45 jack used for data communication wiring.