IEEE Spectrum February, 2013 - 34

caLLing
ma BeLL...
coLLect!
This assortment of
blue boxes from the
1970s mirrors standard
telephone design,
which underwent a
transition from dials to
touch-tone buttons.

Now that New York knows the number you want to call, it makes
the local connection and the directory assistance operator's telephone starts to ring. Up until now everything that has happened has
been perfectly normal, just like Ma Bell intended. But now you, using
Barclay's hack, insert yourself into the process. Before the operator can
answer, you-naughty you-hold a speaker up to your phone's mouthpiece and play your own 2600-Hz tone down the line for a second.
It is loud and pure, and it sounds like this: "Bleeeeeeep."
Seattle isn't paying any attention to this, but the switching machine in New York sure is. New York hears your 2600-Hz tone
loud and clear and thinks that the Seattle switching machine sent
it. And since this tone indicates the trunk line is idle, New York figures that Seattle is done using that trunk line, probably
because you hung up. New York disconnects the call to
the directory assistance operator-maybe before she's
even answered.
But now you stop sending your tone. When you stop
sending 2600 Hz, the long-distance switching equipment
in New York City now thinks that Seattle wants to make
another call. Just like before, New York sends a wink back
to Seattle to say that it's ready for a new call. Due to the
nature of the circuitry involved, the wink has a bright, metallic, ringing quality to it. It sounds like this: "Kerchink!"
That noise tells you that you have just fooled New York
into thinking that a new long-distance call is coming in.
Once again, the switching machine in New York is waiting for Seattle to tell it digits to dial. But Seattle isn't going
to tell it anything, because Seattle is blissfully unaware
of everything that has just transpired. The only thing
Seattle knows is that you haven't hung up-you're still
on the line, after all-and Seattle believes you can only
make one call every time you pick up the phone. As far
as Seattle is concerned, you're still talking to New York
directory assistance.
You, on the other hand, know better: You possess guilty
knowledge. Using a simple little electronic circuit, you
can generate the same pairs of tones that Ma Bell's telephone switches use to serenade each other. Once again
34

fEb 2013

NORTh AMERICAN

SPECTRUM.IEEE.ORG

holding up a speaker to your phone, you play the tones needed
to send New York the digits KP + 303 722 7209 + ST-that is, the
number of your friend Bill in Denver. Now of course, area code
303 isn't in New York City, but that's okay: The telephone switch
in New York is a brainy #4A and knows how to route calls from
one place to another-after all, Bell Labs worked hard to give it the
brains to be able to do that. New York happily finds a trunk line
to Denver and puts your call through, sending out tones on your
behalf to instruct Denver on what number to dial. Moments later,
Bill's phone starts to ring.
Congratulations: You've just hijacked a phone call to directory
assistance in New York and rerouted it to Bill in Denver. But

The late
Ralph Barclay
shows off his
box in 2009.

http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum February, 2013