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.