Second way you mentioned is the prefered way to call a WCF service. You need to create a client first and call the method from the client,
EndpointAddress _endpoint = new EndpointAddress("net.tcp://" + szWord_Operation_Ip + "/CalcService");
using (Process_Word_Document.CalcServiceClient _client = new Process_Word_Document.CalcServiceClient(_binding, _endpoint))
_client.Update_Document_Properties(szFilePath)