The North American Numbering Plan

Fuld-modell-frankfurtToday is another in the series of blogs that looks at the important developments in the history of the telecom industry as I look at the North American Numbering Plan (NANP). This may seem a  bit mundane, but it’s one of those things that has been done correctly and that has made it easy for the industry to grow and to adapt to changes quickly. NANP was originally operated directly by AT&T, but over time the administration was given first to Lockheed Martin and then to Neustar, which still administers it today.

The NANP was first devised in the 1940’s and introduced in 1947. It replaced a hodgepodge of different numbering schemes in different parts of the US. The numbering plan was expanded over the years to include Canada and much of the Caribbean. The first numbering plan assigned the first three digits of a phone number to Numbering Plan Areas (NPAs) now just called area codes. There was originally room for 152 area codes with 86 of them originally assigned, each of which could contain up to 540 central offices. Area codes were assigned to minimize the number of clicks on a rotary dial phone for the largest number of people. And so New York City, as the most populous place got area code 212 since that was the least number of rotary clicks that was available and that didn’t start or end with a 1.

Area codes made it easier for operators to place long distance calls. They could quickly tell by the phone number where a call was supposed to route rather than having to know the location of every small town in America. The first direct-dialed long distance call was made on November 10, 1951 between Englewood NJ and Alameda CA. Direct-dial quickly spread to larger cities and was introduced everywhere by the early 1960s, eliminating the majority of operators.

Digits 4 – 6 of a phone number designated the central office that handled a call, which meant a physical location and a switch in the early days. When facing a shortage of central office codes by the late 1980s Bellcore required that all long distance calls begin with the prefix 1+ to distinguish them from local calls or from operator calls that begin with 0+. This allowed them to assign central office codes that begin with the numbers of 1 or 0 and alleviated the number shortage. .

Starting in the 1990s there was an explosion of the demand for central office codes and telephone numbers, mostly due to the creation of CLECs in the US. Up until that time, when somebody was given a new central office code they got a block of 10,000 numbers even if they were only going to have a few customers. The shortage of codes was addressed with two numbering strategies – splits and overlays. Splits divided a central office code into two or more parts that were assigned to different central offices. Each carrier would get only some portion of the 10,000 numbers. Overlays were implemented as entire new area codes that were overlayed on top of an existing area code which could provide a whole new set of central office codes. The public hated overlays at first because they could no longer automatically know the area code of somebody who lived in their area.

Anywhere there was an area code overlay it became mandatory for callers to dial ten digits because there were now multiple local people sharing the same last seven digits. By the early 1990s it became mandatory everywhere to dial ten digits to make a local call.

By 2000 there was a growing shortage of phone numbers and this was solved by using number pooling. This involved assigning blocks of numbers in groups of 1,000 instead of 10,000. First, this allowed the recovery of a huge number of unused thousand blocks from existing carriers. It also means that there is a greater chance that all numbers will eventually get used. About this same time the FCC mandated number portability. This allowed customers to keep their number when moving in some situations. At first a customer could keep a number if they stayed within the same local calling scope. But over time number portability has been expanded and includes a portability between landlines and cellular phones. Cellular phones are completely geographically portable and you can take a cell number anywhere. But there are still some restrictions on geographically moving landline numbers.

It is currently estimated that we have enough telephone numbers to last until sometime in the 2030s. At that time it will be necessary to go to 11-digit dialing. It is amazing to me how gracefully the system has changed over the years as they ran out of area codes, central office codes and then numbers. It’s easy to discount the value of the NANP, but you have to admire how it has been able to accommodate the need for over 100 million new cell phone numbers over the last decade. NANP was devised in a deliberate fashion but has been flexible enough to accommodate huge changes in the industry that could not have been contemplated in 1947.

Leave a Reply