Magento call observer on checkout onepage checkout save billing as below.
Syntax of Magento call observer on checkout onepage checkout
Add the following lines of code in your config.xml with details of your module.
<events>
<controller_action_postdispatch_checkout_onepage_savebilling>
<observers>
<namespace_modulename_model_observer>
<type>singleton</type>
<class>Namespace_ModuleName_Model_Observer</class>
<method>Name_of_method</method>
</namespace_modulename_model_observer>
</observers>
</controller_action_postdispatch_checkout_onepage_savebilling>
</events>