public static function isPhoneNumber($value) {
        if (!is_string($value)) return false;

        // handle empty value
        $value = trim($value);
        if (!strlen($value)) return false;

        // remove spaces
        $value = str_replace(' ', '', $value);

        // match numbers from BE, ES, BG, CH, A, UK, DE
        return (bool) preg_match('/^(00|\+)(32|34|359|41|43|44|49)-?[1-9](-?[0-9]){2,}$/', $value);

        // TODO: implement via https://github.com/giggsey/libphonenumber-for-php
    }