<input type="text" class="small" name="in_tel" id="in_tel" />
-----------------------------------------
$this->load->library('form_validation');
$this->form_validation->set_rules('in_tel', 'Telefon', 'trim|callback_OzgunDogrulamaKurali|xss_clean');
if ($this->form_validation->run() == FALSE)
{   //Yanlis doldurulmus form... }  
else 
{   //Dogru doldurulmus form... }  

public function OzgunDogrulamaKurali($rData) 
{   //Istediginiz dogrulama sartlari... }