The Phreaker's Guide to Loop Lines courtesy of the Jolly Roger

A loop is a wonderous device which the telephone company created as test numbers for telephone repairmen when testing equipment. By matching the tone of the equipment with the tone of the loop, repairmen can adjust and test the settings of their telephone equipment.

A loop, basically, consists of two different telephone numbers. Let's use A and B as an example. Normally if you call A, you will hear a loud tone (this is a 1004 hz tone), and if you call B, the line will connect, and will be followed by silence.

This is the format of a loop line. Now, if somebody calls A and someone else calls B--Viola!--A and B loop together, and one connection is made. Ma Bell did this so repairmen can communicate with each other without having to call their own repair office. They can also use them to exchange programs, like for ANA or Ringback. Also, many CO's have a "Loop Assignment Center". If anyone has any information on these centers please tell me.

Anyway, that is how a loop is constructed. From this information, anyone can find an actual loop line. Going back to the A and B example, Note: the tone side and the silent side can be either A or B. Don't be fooled if the phone company decides to scramble them around to be cute.

As you now know, loops come in pairs of numbers. Usually, right after each other. For example:
817-972-1890 and 817-972-1891

Or, to save space, one loop line can be written as 817-972-1890/1. This is not always true. Sometimes, the pattern is in the tens or hundreds, and, occaisionally, the numbers are random.

In cities, usually the phone company has set aside a phone number suffix that loops will be used for. Many different prefixes will correspond with that one suffix.

In Arlington, Texas, a popular suffix for loops is 1893 and 1894, and a lot of prefixes match with them to make the number. For Example:

or a shorter way to write this is: 
              817-xxx-1893/4
      xxx= 460, 461, 465, 467, 469
Note: You can mix-and-match a popular suffix with other prefixs in a city, and almost always find other loops or test numbers.
Note: For Houston, the loop suffixes are 1499 and 1799. And for Detroit it's 9996 and 9997.
When there are a large number of loops with the same prefix format, chances are that many loops will be inter-locked. Using the above example of Arlington loops again, (I will write the prefixes to save space) 460, 461, and 469 are interlocked loops. This means that only one side can be used at a given time. This is because they are all on the same circuit.

To clarify, if 817-461-1893 is called, 817-460 and 469-1893 cannot be called because that circuit is being used. Essentialy, interlocked loops are all the same line, but there are a variety of telephone numbers to access the line.

Also, if the operator is asked to break in on a busy loop line he/she will say that the circuit is overloaded, or something along those lines. This is because Ma Bell has taken the checking equipment off the line. However, there are still many rarely used loops which can be verfied and can have emergency calls taken on them.

As you have found out, loops come in many types. Another type of loop is a filtered loop. These are loop lines that the tel co has put a filter on, so that normal human voices cannot be heard on either line. However, other frequencies may be heard. It all depends on what the tel co wants the loop to be used for. If a loop has gotten to be very popular with the local population or used frequently for conferences, etc. the tel co may filter the loop to stop the unwanted "traffic". Usually, the filter will be removed after a few months, though.

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