 One advantage is that, during the transition period, permissive dialing could be enabled.This means that until everyone has adjusted to the new dialing system, users would still be able to dial the shorter, 10-digit numbers.Since currently the 4th digit (or digit 'D') cannot be 1 or 0, if the telephone system detects 1 or 0 in the 4th position it will process the number as a new 12-digit number, and if it is any digit other than 1 or 0, it can process it as an existing 10-digit number until the transition is complete.