invoke web service within a database function

Hi all,
We have the following function to call a web service:

CREATE OR REPLACE FUNCTION add_numbers_test_cartable  RETURN varchar2
AS
  l_service          UTL_DBWS.service;
  l_call             UTL_DBWS.call;

  l_wsdl_url         VARCHAR2(32767);
  l_namespace        VARCHAR2(32767);
  l_service_qname    UTL_DBWS.qname;
  l_port_qname       UTL_DBWS.qname;
  l_operation_qname  UTL_DBWS.qname;

  l_xmltype_in       SYS.XMLTYPE;
  l_xmltype_out      SYS.XMLTYPE;
  l_return           NUMBER;
BEGIN

  l_wsdl_url        := 'http://mapor-soa.mapna.com:8001/soa-infra/services/default/CartableProject/LetterProcess.service?WSDL';

   l_namespace       :='http://xmlns.oracle.com/bpmn/bpmnProcess/LetterProcess';

  l_service_qname   := UTL_DBWS.to_qname(l_namespace, 'LetterProcess.service');