Similarly, once we did away with legacy electromechanical phone switches, the (phone area code) NANPA started allowing area codes with non-[0,1] middle digits.

Similarly, once we did away with legacy electromechanical phone switches, the (phone area code) NANPA started allowing area codes with non-[0,1] middle digits. The big form of 10-digit numbers was retained and extended by relaxing a legacy constraint.