This basic question for many people that how to call core phtml file into custom module. Before few days i have no answer. But today i have. If you want to call any custom file into for custom module's phml file you can do it as following.

In this example i call core list.phml into my custom module file.

<catalog_category_layered translate="label">
        <reference name="product_list">
            <action method="setTemplate">
                <template>customoption/customoption.phtml</template>
            </action>
        </reference>
</catalog_category_layered>

See how it works. customoption is my module name and customoption.phtml is my module phml file. And i replaced my default magento list.phml file with my module's phtml file. So suppose if you open your list page it will saw what ever in my customoption.phtml.