[2] As a result, the North American telephone administrations first introduced letter combinations that could not be linked to a familiar pronounceable central office name in some highly populated states, such as New York.
The final solution to the growing threat of numbering exhaustion was decided by AT&T engineers and administrators from in-depth studies of all alternative methods.
In 1954, John Karlin directed a research project that investigated the memory capacity and dialing accuracy of employees when using seven-digit telephone numbers comprising only digits.
The practice did not produce any adverse effects,[4] and opened the path for listing telephone numbers in the 2-5 style, where the two letters were unrelated to any pronounceable name.
[2] This increased the numbering pool for central office codes to 640, and resulted in the partitioning of the prefix space (000—999) according to the table at the right.
[4] During larger scale introductions in California in 1962, this change sparked an intense outcry among urban users who considered all-digit dialing to be dehumanizing.