Almost all 'extra' data for any entity in Elgg is stored in the metadata table (http://docs.elgg.org/wiki/DatabaseSchema)

in your example the phonenumber would be store in that table.

If you want to search a user by phonenumber have a look at the function elgg_get_entities_from_metadata

$options could look something like

array(

  'type' => 'user',

  'metadata_name_value_pairs' => array(

     'name' => 'phone',

     'value' => '%$some_var%'

     'operand' => 'LIKE'

  )

)

Thanks Jerome Bakker!!! It worked perfectly! 

Jerome, How can I get all users + phones?

@Thiago Melo:

How can I get all users + phones?

Assuming phonenumbers are stored in the field "phone"

echo $user->phone;

Or if you wish to use it somewhere else

$result = array();

foreach ($users as $user) {

   $result[$user->username] = $user->phone;

   or

   $result[] = array($user->name, $user->phone);

}

Thanks very much @Jerome Bakker!