Try to add this code snippet to the functions.php file in your child theme

add_action('woocommerce_single_product_summary', 'my_text_after_button',38);
function my_text_after_button() {
?>
<div class=”place_order_by_phone”>
<b>Place Order By Phone:</b> <span class=”phone_icon”>+91-261-2507700</span> <span class=”whatsapp_icon”>+91-7285880241</span>
</div>
<?php
}