// You can add a custom placeholder to add a hint for your CUs what you expect
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields2' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields2( $fields ) {
	
	$fields['billing']['billing_phone']['placeholder'] = '09XXXXXXXXX';

    return $fields;
}

/****************************************************************/
/* VALIDATION FOR PHONE FIELD THIS WILL THROW AN ERROR MESSAGE  */
/****************************************************************/

/**
 * Process the checkout
 **
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
	global $woocommerce;

	// Check if set, if its not set add an error. This one is only requite for companies
	if ( ! (preg_match('/^[0-9]{10}$/D', $_POST['billing_phone'] ))){
		wc_add_notice( "The Phone should contain only 10 digits"  ,'error' );
	}
	
}