How Old Phone Systems Knew your Number


How did central offices know your phone number?

This sounds like a stupid, obvious question. But it was remarkably complicated.

Step by step switches don’t know the phone number you are calling from, which is why they could not handle any part of processing long distance phone calls. End of story. (this is why flat-rate service was the norm on the west coat: no way to charge you for local calls in a step-by-step switch.)

Panel and Crossbar offices could bill you for calls because they knew your phone number. But it’s way more complicated…

Your phone line is wired to a terminal called a “Line Equipment Number” or LEN. This LEN is not a phone number and cannot be dialed. Think of it as something like an IP address. It is just a unique sequential number your line is hooked up to. So, so far you don’t have a phone number.

You were given a phone number by taking a pair of wires from your LEN and stringing them through a series of loops! In a central office there would be loops 1 through 0 for the first digit, and another set of loops 1 through 0 for the second digit, and so on. All in all, 70 loops existed to support the ability to have one loop for each possible digit. (There could be fewer since the first 2 or 3 digits could often be pre-known since they were the same for everybody in the switch.)

So, when your phone line was installed, the installer would run this extra pair from your LEN through each successive loop to create your phone number.

Now for the magic!

When the central office needed to know your phone number (such as for a long distance call) it would put a very strong pulse on your phone line. This pulse would get picked up in each of the loops your line was routed through. (The loops actually created an inductive pickup, similar to a transformer.) Presto! In a small fraction of a second all of your phone number digits were identified at one time!

There were some implications of this system. If your wire was mis-routed, then all of your long distance calls would be billed to the incorrectly wired number. If one of the vacuum tubes of the loop detector failed, then it might not register your number, or might mis-register it. And, multiple LEN numbers could share a single phone number.

These detector loops served one other very important function: An extra set of loops could be used to give your line a “class of service”. This means that you could be denied long distance, or you could be given a feature, or all of your calls could be traced, or whatever by routing your line through one of the class of service loops.

All of this is remarkable for a system designed in the 1920s. These “Panel” central offices were utterly amazing, decades ahead of their time, and they had a serving life as long as fifty years. As new features were needed they could be added easily using class of service and the relay-based computer.

Colin Berkshire