$ship_address = new Address($this->context->cart->id_customer);
$this->context->smarty->assign(array(

'stf_phone' => $ship_address->phone

));

<input id="friend_phone" name="friend_phone" type="text" value="{$stf_phone}"/>

if ($this->context->customer->isLogged() && isset($this->context->cart->id_address_delivery))
{
    $ship_address = new Address($this->context->cart->id_address_delivery);
    if($ship_address->phone || $ship_address->phone_mobile)
        $this->context->smarty->assign('stf_phone', ($ship_address->phone ? $ship_address->phone : $ship_address->phone_mobile));
}