How do I retrieve the call-log content or mp3?

Are you trying to retrieve call recordings in Production Environment ?  If so please be aware that the Production Environment does not support accessing call recordings through API's.

function getURL($url, $login = false){
        $ch = curl_init ($url);
        
        if($login)
        {
                $poststring = 'LoginName='.$Username.
                              '&country=2'.
                              '&Password='.$Password;
                curl_setopt ($ch, CURLOPT_POST, 1);
                curl_setopt ($ch, CURLOPT_POSTFIELDS, $poststring);
        }
        
        curl_setopt($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.12) Gecko/20050915 Firefox/1.0.7");
        curl_setopt ($ch, CURLOPT_COOKIEJAR, 'RC_Cookie.txt');
        curl_setopt ($ch, CURLOPT_COOKIEFILE, 'RC_Cookie.txt');
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
        $data = curl_exec ($ch);
        curl_close($ch);
        return $data;
}

$Username = '1713XXXXXXX';
$Password = 'XXX';

getURL('https://service.ringcentral.com/login/main.asp',true);

file_put_contents('data.mp3',getURL('https://service.ringcentral.com/mobile/media?cmd=downloadMessage&msgid='.$MessageID));

There's no need. I feel like the script I wrote is much better. If I were to use the API calls, I would have to throttle the amount of times I hit your servers while if I download each call recording using my script, I'm going to guess I can bypass the too many request API error and download each audio file one after the other.