[System.ServiceModel.FaultContractAttribute(typeof(FaultClass[]), Action = "http://whatever/", Name = "whateverBusinessFault")]

try
{
  // call service here
}
catch (FaultException<FaultClass[]> ex)
{
  if (ex.Detail != null && ex.Detail.Length > 0)
  {
    throw new CustomException(ex.Detail[0].description);
  }
  else
  {
    throw;
  }
}

var sb = new StringBuilder();

using (XmlReader reader = fault.GetReaderAtDetailContents())
{
  while (reader.Read())
     sb.AppendLine(reader.ReadOuterXml());
}

var detail = sb.ToString();

try
{
    proxy.CallSomeMethod();
}
catch (FaultException ex)
{
    var fault = ex.CreateMessageFault();
    using (XmlReader reader = fault.GetReaderAtDetailContents())
    {
        // TODO: read the XML fault and extract the necessary information.
    }
}

[ServiceContract]
public interface IService1
{
    [OperationContract]
    [FaultContract(typeof(FaultClass[]))]
    string Crash();
}

public class Service1 : IService1
{
    public string Crash()
    {
        var exception = new FaultException<FaultClass[]>(new FaultClass[] { new FaultClass { Data = "TEST" } }, new FaultReason("Boom"));

        throw exception;
    }
}

[DataContract]
public class FaultClass
{
    [DataMember]
    public string Data { get; set; }
}

try
{
    using (var client = new Service1Client())
    {
        client.Crash();
    }
}
catch(FaultException<FaultClass[]> e)
{
    //Break here
}

public static string GetDetail(this FaultException faultException)
{
    if (faultException == null)
        throw new ArgumentNullException(nameof(faultException));

    MessageFault messageFault = faultException.CreateMessageFault();
    if (messageFault.HasDetail) {
        using (XmlDictionaryReader reader = messageFault.GetReaderAtDetailContents()) {
            return reader.ReadContentAsString();
        }
    }
    return null;
}