FedEx Sandbox has problems, it is a known issue with developers that integrate with FedEx APIs. The issue you have is one of many.

To resolve the issue with FedEx Sandbox environment stability you have three options, which are essentially about using test-doubles instead of the real FedEx API:

• Create and use mocks in your code instead of using the FedEx Sandbox. For example, Mockito in Java. This means you will have to write the FedEx Mockito mocks yourself. And it comes with several tradeoffs.
• Create over-the-wire mocks with tools like Wiremock instead of using the FedEx Sandbox. There is a list of tools like that on Wikipedia. The issue here again is that you have to build those over-the-wire mocks yourself.
• Use ready-made FedEx API mocks included in ready-made Sandbox environments like the one from Traffic Parrot instead of using the FedEx Sandbox. They will be ready to use within minutes, but you have to pay for it (the current price listed is "starting at $50/month").

fedex offers acceleration packages in www.fedex.com/us/developer/ , you will find information about different types of calls to their webservices. as an example if you want to request a rate from fedex you will need to do something like this:

<?php

require_once('../../library/fedex-common.php5');

$newline = "<br />";
//The WSDL is not included with the sample code.
//Please include and reference in $path_to_wsdl variable.
$path_to_wsdl = "../../wsdl/RateService_v13.wsdl";

ini_set("soap.wsdl_cache_enabled", "0");

$client = new SoapClient($path_to_wsdl, array('trace' => 1)); // Refer to http://us3.php.net/manual/en/ref.soap.php for more information

$request['WebAuthenticationDetail'] = array(
    'UserCredential' =>array(
        'Key' => getProperty('key'), 
        'Password' => getProperty('password')
    )
); 
$request['ClientDetail'] = array(
    'AccountNumber' => getProperty('shipaccount'), 
    'MeterNumber' => getProperty('meter')
);
$request['TransactionDetail'] = array('CustomerTransactionId' => ' *** Rate Request v13 using PHP ***');
$request['Version'] = array(
    'ServiceId' => 'crs', 
    'Major' => '13', 
    'Intermediate' => '0', 
    'Minor' => '0'
);
$request['ReturnTransitAndCommit'] = true;
$request['RequestedShipment']['DropoffType'] = 'REGULAR_PICKUP'; // valid values REGULAR_PICKUP, REQUEST_COURIER, ...
$request['RequestedShipment']['ShipTimestamp'] = date('c');
$request['RequestedShipment']['ServiceType'] = 'INTERNATIONAL_PRIORITY'; // valid values STANDARD_OVERNIGHT, PRIORITY_OVERNIGHT, FEDEX_GROUND, ...
$request['RequestedShipment']['PackagingType'] = 'YOUR_PACKAGING'; // valid values FEDEX_BOX, FEDEX_PAK, FEDEX_TUBE, YOUR_PACKAGING, ...
$request['RequestedShipment']['TotalInsuredValue']=array('Ammount'=>100,'Currency'=>'USD');
$request['RequestedShipment']['Shipper'] = addShipper();
$request['RequestedShipment']['Recipient'] = addRecipient();
$request['RequestedShipment']['ShippingChargesPayment'] = addShippingChargesPayment();
$request['RequestedShipment']['RateRequestTypes'] = 'ACCOUNT'; 
$request['RequestedShipment']['RateRequestTypes'] = 'LIST'; 
$request['RequestedShipment']['PackageCount'] = '1';
$request['RequestedShipment']['RequestedPackageLineItems'] = addPackageLineItem1();
try 
{
    if(setEndpoint('changeEndpoint'))
    {
        $newLocation = $client->__setLocation(setEndpoint('endpoint'));
    }

    $response = $client ->getRates($request);

    if ($response -> HighestSeverity != 'FAILURE' && $response -> HighestSeverity != 'ERROR')
    {   
        $rateReply = $response -> RateReplyDetails;
        echo '<table border="1">';
        echo '<tr><td>Service Type</td><td>Amount</td><td>Delivery Date</td></tr><tr>';
        $serviceType = '<td>'.$rateReply -> ServiceType . '</td>';
        $amount = '<td>$' . number_format($rateReply->RatedShipmentDetails[0]->ShipmentRateDetail->TotalNetCharge->Amount,2,".",",") . '</td>';
        if(array_key_exists('DeliveryTimestamp',$rateReply)){
            $deliveryDate= '<td>' . $rateReply->DeliveryTimestamp . '</td>';
        }else if(array_key_exists('TransitTime',$rateReply)){
            $deliveryDate= '<td>' . $rateReply->TransitTime . '</td>';
        }else {
            $deliveryDate='<td>&nbsp;</td>';
        }
        echo $serviceType . $amount. $deliveryDate;
        echo '</tr>';
        echo '</table>';

        printSuccess($client, $response);
    }
    else
    {
        printError($client, $response);
    } 

    writeToLog($client);    // Write to log file   

} catch (SoapFault $exception) {
   printFault($exception, $client);        
}

function addShipper(){
    $shipper = array(
        'Contact' => array(
            'PersonName' => 'Sender Name',
            'CompanyName' => 'Sender Company Name',
            'PhoneNumber' => '9012638716'),
        'Address' => array(
            'StreetLines' => array('Address Line 1'),
            'City' => 'Collierville',
            'StateOrProvinceCode' => 'TN',
            'PostalCode' => '38017',
            'CountryCode' => 'US')
    );
    return $shipper;
}
function addRecipient(){
    $recipient = array(
        'Contact' => array(
            'PersonName' => 'Recipient Name',
            'CompanyName' => 'Company Name',
            'PhoneNumber' => '9012637906'
        ),
        'Address' => array(
            'StreetLines' => array('Address Line 1'),
            'City' => 'Richmond',
            'StateOrProvinceCode' => 'BC',
            'PostalCode' => 'V7C4V4',
            'CountryCode' => 'CA',
            'Residential' => false)
    );
    return $recipient;                                      
}
function addShippingChargesPayment(){
    $shippingChargesPayment = array(
        'PaymentType' => 'SENDER', // valid values RECIPIENT, SENDER and THIRD_PARTY
        'Payor' => array(
            'ResponsibleParty' => array(
            'AccountNumber' => getProperty('billaccount'),
            'CountryCode' => 'US')
        )
    );
    return $shippingChargesPayment;
}
function addLabelSpecification(){
    $labelSpecification = array(
        'LabelFormatType' => 'COMMON2D', // valid values COMMON2D, LABEL_DATA_ONLY
        'ImageType' => 'PDF',  // valid values DPL, EPL2, PDF, ZPLII and PNG
        'LabelStockType' => 'PAPER_7X4.75');
    return $labelSpecification;
}
function addSpecialServices(){
    $specialServices = array(
        'SpecialServiceTypes' => array('COD'),
        'CodDetail' => array(
            'CodCollectionAmount' => array('Currency' => 'USD', 'Amount' => 150),
            'CollectionType' => 'ANY')// ANY, GUARANTEED_FUNDS
    );
    return $specialServices; 
}
function addPackageLineItem1(){
    $packageLineItem = array(
        'SequenceNumber'=>1,
        'GroupPackageCount'=>1,
        'Weight' => array(
            'Value' => 50.0,
            'Units' => 'LB'
        ),
        'Dimensions' => array(
            'Length' => 108,
            'Width' => 5,
            'Height' => 5,
            'Units' => 'IN'
        )
    );
    return $packageLineItem;
}

?>

so go to fedex.com, download wsdl or xml with library and more. run this code and you will receive a quote. important to say that you need an account to access that area, where you will receive a test meter-account to try, and then move to production.. hope it helps.