Probeer deze code:
// Create an array of the values to use in the list
$villes = array("paris", "fes", "rabat");
// Generate the SQL statement.
// The number of %s items is based on the length of the $villes array
$sql = "
SELECT DISTINCT telecopie
FROM `comptage_fax`
WHERE `ville` IN(".implode(', ', array_fill(0, count($villes), '%s')).")
";
// Call $wpdb->prepare passing the values of the array as separate arguments
$query = call_user_func_array(array($wpdb, 'prepare'), array_merge(array($sql), $villes));
echo $query;