<?xml version="1.0"?>
<config>
<modules>
    <Custom_Module>
        <active>true</active>
        <codePool>local</codePool>
    </Custom_Module>
</modules>
</config>

<config>
<modules>
    <Custom_Module>
        <version>0.1.2</version>
    </Custom_Module>
</modules>
<events>
<checkout_onepage_controller_success_action>
    <observers>
        <send_sms_after_placeorder>
           <class>smsgateway/observer</class>
           <method>sendPlaceOrderSms</method>
       </send_sms_after_placeorder>
    </observers>
</checkout_onepage_controller_success_action>
</events>
</config>

public function sendPlaceOrderSms(Varien_Event_Observer $observer)
{
    $orderIds = $observer->getData('order_ids');
    foreach($orderIds as $_orderId)
    {
        $order = Mage::getModel('sales/order')->load($_orderId);
        $items = $order->getAllItems();
        $order_increment_id = $order->getIncrementId();
        $get_billing_mobile = $order->getBillingAddress()->getTelephone();
        foreach ($items as $cartItem)
        {
            if($cartItem->getData('product_type')== 'configurable'){
                $product_name = $cartItem->getName();
            }
        }
    }
    $URL = 'your api url';
    $USERNAME = 'api username';
    $PASSWORD = 'api password';
    $USERID = 'api userid';
    $PHONE = $get_billing_mobile;
    $message = "Your Order # $order_increment_id for $product_name.. has been successfully placed. It should be delivered within 3 to 5 days. Thank you for shopping at example.com";
    $TXT = urlencode ($message);
    $Q = "$URL?user=$USERNAME&pass=$PASSWORD&sender=$USERID&phone=$PHONE&text=$TXT&priority=ndnd&stype=normal";
    $res = implode ('', file ($Q));
}