follow this: https://learn.microsoft.com/en-us/answers/questions/712472/c-parse-soap-response-for-elements-and-attribute-v

var faultElement = responseXmlDoc.Descendants(soapNamespace + "Fault").FirstOrDefault();
string faultCode = responseXmlDoc.Descendants("faultcode").FirstOrDefault()?.Value;
string faultString = responseXmlDoc.Descendants("faultstring").FirstOrDefault()?.Value;