In response I always receive message that echo parameter is not given, so I wrote a simple soap client using PHP (response is correct).
I'm sure the problem only affects xml request (not header), because I injected PHP xml request statically in QtSoap plugin (reponse was correct).

PHP xml request:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="urn:StWebApi" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
   <SOAP-ENV:Body>
      <ns1:test>
         <test xsi:type="ns1:test">
            <echo xsi:type="xsd:string">aaaaaaaaaaaaaaddd</echo>
         </test>
      </ns1:test>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

C++ xml request:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/1999/XMLSchema" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
   <SOAP-ENV:Body xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
      <test>
         <echo xmlns:xsi="http://www.w3.org/1999/XMLSchema-instance" xsi:type="xsd:string">aaaaaaaaaaaaaaddd</echo>
      </test>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

C++ code:

Test::Test(QObject* parent) :
    QObject(parent)
{
    connect(&http, SIGNAL(responseReady()), SLOT(getResponse()));
    http.setAction("urn:StWebApiSoapServer");
    http.setHost("mywebpage.com", true);
}

Test::~Test()
{
}

void Test::sendRequest()
{
    QtSoapMessage request;
    request.setMethod(QtSoapQName("test", ""));
    request.addMethodArgument("echo", "", "aaaaaaaaaaaaaaddd");

    // Submit the method request to the web service.
    http.submitRequest(request, "/backend.php/webapi/soap");
}

void Test::getResponse()
{
    // Get the response, check for error.
    const QtSoapMessage &resp = http.getResponse();
    if (resp.isFault())
    {
        qDebug() << (tr("Query failed: ")
                    + resp.faultString().value().toString());
        return;
    }

    // Extract the return value from this method response, check for
    // errors.
    const QtSoapType &res = resp.returnValue();
    if (!res.isValid())
    {
        qDebug() << (tr("Invalid return value"));
        return;
    }
    qDebug() << "response: " << resp.toXmlString();
            return;
}

As You can see xml requests are a bit different.
What's wrong with my code?