/**
 * Validates if a string is a valid phone number according to specified format.
 * @param {string} phone - The phone number to validate.
 * @param {'E164' | 'NANP'} format - The format to validate against (E164 or North American Number Plan).
 * @returns {boolean} True if the phone number is valid, false otherwise.
 * @example
 * // E164 format
 * console.log(isPhoneNumber('+14155552671')); // true
 * console.log(isPhoneNumber('+1415555267')); // false
 *
 * // NANP format
 * console.log(isPhoneNumber('(415) 555-2671', 'NANP')); // true
 * console.log(isPhoneNumber('415-555-2671', 'NANP')); // true
 * console.log(isPhoneNumber('4155552671', 'NANP')); // true
 */
export declare function isPhoneNumber(phone: string, format?: 'E164' | 'NANP'): boolean;