<InvoiceTransmission xmlns="http://www.IATA.com">
...
</InvoiceTransmission">

SELECT a.xmlns FROM XMLT,
XMLTABLE('/InvoiceTransmission'
PASSING XMLT.data COLUMNS
xmlns VARCHAR2(50) PATH '@xmlns'
)a;

WITH xmlt AS 
(SELECT xmltype(
	'<InvoiceTransmission xmlns="http://www.IATA.com">
	<data>info</data>
	</InvoiceTransmission>'
) data FROM dual)
SELECT xmlns FROM XMLT,
XMLTABLE('/InvoiceTransmission'
PASSING XMLT.data COLUMNS
xmlns VARCHAR2(50) PATH '@xmlns'
)