The first digit of N-1-1 codes can be any digit other than:

The first digit of N-1-1 codes, which are reserved for special services such as emergency numbers, cannot be '0'. This is because the numbering plan for N-1-1 codes was designed to avoid confusion with regular seven-digit telephone numbers and other dialing patterns.

Typically, for the North American Numbering Plan, the first digit of a three-digit code serves a specific purpose and must conform to certain standards. By convention, digits such as '1' and '9' are often valid first digits in various numbering plans, and '5' is also permissible in certain contexts. However, using '0' as the first digit of these special codes would create ambiguity, as it is typically not used in the beginning of any area code or standard telephone number format.

Thus, the restriction on '0' as the first digit ensures clarity and maintains consistency across the dialing systems for emergency and service codes, making this the correct answer.