def validate_imei(imei: str) -> bool:
    """
    Function to validate the International Mobile Equipment Identity (IMEI) number.
 
    IMEI is a unique identifier for mobile devices, consisting of 15 digits.
    The last digit is a check digit calculated using the Luhn algorithm.
 
    Parameters:
    - imei: str
        The IMEI number to be validated.
 
    Returns:
    - bool:
        True if the IMEI is valid, False otherwise.
 
    Raises:
    - ValueError:
        Raises an error if the IMEI is not a string or if it does not consist of exactly 15 digits.
    """
 
    # Checking if the IMEI is a string and consists of exactly 15 digits
    if not isinstance(imei, str) or not imei.isdigit() or len(imei) != 15:
        raise ValueError("IMEI should be a string consisting of exactly 15 digits.")
 
    # Calculating the check digit using the Luhn algorithm
    check_digit = int(imei[-1])
    imei_digits = [int(digit) for digit in imei[:-1]]
    imei_digits.reverse()
 
    for i in range(len(imei_digits)):
        if i % 2 == 0:
            imei_digits[i] *= 2
            if imei_digits[i] > 9:
                imei_digits[i] -= 9
 
    calculated_check_digit = (10 - sum(imei_digits) % 10) % 10
 
    # Validating the check digit
    return check_digit == calculated_check_digit
 
# Example usage:
imei_number = "123456789012345"
is_valid = validate_imei(imei_number)
print(f"The IMEI number {imei_number} is valid: {is_valid}")

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