With the following code:
$pn[] = 'APPSP2101V2';
$pn[] = 'ME665Y/A';
$PartNumbers = array( 'PartNumbers' => array('string' => $pn));
$result = $client->call('GetDataSheetsLastUpdate', $PartNumbers );
Works perfect , the problema was in the webservice.. if the P/N isn't correct only return the P/N with information on it.