the shipping country is stored in a sessin variable named:
eCartCheckoutForm_shipping_country
add an if statement around the checkout button to only show if the shipping country is US:
<?php if(isset($_SESSION['"eCartCheckoutForm_shipping_country']) && $_SESSION['"eCartCheckoutForm_shipping_country'] == "US") { ?>
checkout button
<?php } else { ?>
Please phone in your order
<?php } ?>