[1][2] This change in numbering format was implemented first for central office codes by 1973, which eliminated the restriction in the middle digit (2 to 9) to also permit 0 and 1.
The Bell System engineers expected that the first numbering plan areas to require more than the 640 possible central office codes would not occur before c.
[3] The solution for expanding the numbering pool within an NPA was to remove the restriction posed by not using 0 and 1 in the middle position of central office codes.
Canadian telephone companies made the change in Fall 1994 outside of area code 905, which had already implemented the requirement (before the split from 416) in 1990.
The introduction of interchangeable codes required new technology for distinguishing a subscriber's intent of dialing seven or ten digits.
The preferred arrangement, over timing, was the prefix method, in which the subscriber had to start dialing ten-digit calls with a 0 or 1 preceding the full telephone number.
The preference was based on Bell System behavioral research, but technical aspect also favored this arrangement in some cases, in that it could shorten post-dial delay with new equipment.