API Search

Hi

I want to search for a value in a defined field(user_number_c) in one module(contacts).

The only function i found was search_by_module.

I tired the following code with no sucess:

$parameters = array(
		'session' => $crmsession,
		'search_string' => 'user_numer_c:1535',
		'modules' => array('contacts',),
		'offset' => 0,
		'max_results' => 2,
		'id' => '',
		'select_fields' => array('id',),
		'unified_search_only' => false,
		'favorites' => false		
	);
	
	$result = callAPI('search_by_module',$parameters); -> no result records



$parameters = array(
		'session' => $crmsession,
		'search_string' => '1535',
		'modules' => array('contacts',),
		'offset' => 0,
		'max_results' => 2,
		'id' => '',
		'select_fields' => array('id',),
		'unified_search_only' => false,
		'favorites' => false		
	);
	
	$result = callAPI('search_by_module',$parameters); -> to many result records because values from others field do also match

Does somebody know how to create a search function to get this working?

Best Regards
klippit