Hi,
If you want to change the text of the button, you can use a translation override:
www.hikashop.com/download/languages.html#modify
If you want to remove the add to cart button and add instead another button which does something else, the best is to do this:
Use the "Display view files" setting of the HikaShop configuration to see which view file displays the add to cart button.
Add such code at the beginning:
<input type="button" value="Ring now"/>
<?php return; ?>
Of course, you need to modify the code of your new button for it to do the action you want.