Here is the example for shipping and billing phone number validation. Add this custom code to your WordPress site

modify_form_field.php

<?php

// Shipping filed validation
add_filter('cartimize_get_shipping_checkout_fields', 'cartimize_modify_shipping_checkout_fields_new', 100, 1);

function cartimize_modify_shipping_checkout_fields_new( $checkout_fields ){
	if ( isset($checkout_fields['shipping_phone']) && $checkout_fields['shipping_phone']['required'] == true ) {
		$checkout_fields['shipping_phone'] = $checkout_fields['shipping_phone'];
		$checkout_fields['shipping_phone']['custom_attributes']['data-parsley-pattern'] = '([0-9]{10})';
	}
	return $checkout_fields;
}

// Billing filed validation
add_filter('cartimize_get_billing_checkout_fields', 'cartimize_modify_billing_checkout_fields_new', 100, 1);
function cartimize_modify_billing_checkout_fields_new( $checkout_fields ){
	if ( isset($checkout_fields['billing_phone']) && $checkout_fields['billing_phone']['required'] == true ) {
		$checkout_fields['billing_phone'] = $checkout_fields['billing_phone'];
		$checkout_fields['billing_phone']['custom_attributes']['data-parsley-pattern'] = '([0-9]{10})';
	}
	return $checkout_fields;

}

Use below prefix for South Africa phone number validation

^((?:\+27|27)|0)(\d{2})-?(\d{3})-?(\d{4})$

For example

1. Shipping section
   $checkout_fields['shipping_phone']['custom_attributes']['data-parsley-pattern'] = '^((?:\+27|27)|0)(\d{2})-?(\d{3})-?(\d{4})$';

2. Billing section
   $checkout_fields['billing_phone']['custom_attributes']['data-parsley-pattern'] = '^((?:\+27|27)|0)(\d{2})-?(\d{3})-?(\d{4})$';