In North America, the area code 543 is not currently in use. The numerically closest existing area codes are 541 for Eugene, Oregon, and 551 for Newark, New Jersey.
The North American Numbering Plan Association, or NANPA, manages and administers area codes. NPA codes are the actual name for what most people refer to as area codes. Theoretically, every three-digit number (000-999) can be used as an area code, but only 316 of them are currently in use. Furthermore, some codes are kept as unique markers of some kind. The 800 area code, for example, is used for “toll-free” calling, in which the recipient pays rather than the caller.