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<?php

add_action( 'woocommerce_admin_order_data_after_billing_address', 'action_function_name_2485' );
function action_function_name_2485( $order ){
    
    
    $phone = ltrim($order->get_data()['billing']['phone'], '+');
    
    if ( $phone[0] == 8 ) {
        $phone = substr_replace($phone, 7, 0, 1);
    }
    
	foreach ($order->get_meta_data() as $key=>$value) {
	    
	    if ( strpos($value->value, 'Whatsapp') !== false ) {
	        echo "<a href='https://api.whatsapp.com/send?phone=$phone' target='_blank'> Написать в Whatsapp</a>";
	    }
	    
    }
}