public function getDefaultHeaders()
    {
        return [
            'User-Agent' => 'plivo-php/' . implode('.',
                    [Version::MAJOR, Version::MINOR, Version::PATCH])
                    . ' (PHP ' . $this->phpVersion . ')',
            'Accept-Encoding' => '*',
            'Content-type' => 'application/json'
        ];
    }