Create di.xml file in Vendor/Module/etc/di.xml & put below code.
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="Magento\Checkout\Block\Checkout\LayoutProcessor">
<plugin name="validate_telephone_checkout_layoutprocessor" type="Vendor\Module\Plugin\LayoutProcessor" sortOrder="100"/>
</type>
</config>
Now, create LayoutProcessor.php in Vendor\Module\Plugin\LayoutProcessor & put below code
<?php
namespace Vendor\Module\Plugin;
class LayoutProcessor
{
/**
* @param \Magento\Checkout\Block\Checkout\LayoutProcessor $subject
* @param array $jsLayout
* @return array
*/
public function afterProcess(
\Magento\Checkout\Block\Checkout\LayoutProcessor $subject,
array $jsLayout
) {
if (isset($jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
['children']['shippingAddress']['children']['shipping-address-fieldset']['children']
)) {
$jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
['children']['shippingAddress']['children']['shipping-address-fieldset']['children']
['telephone']['validation'] = ['required-entry' => true, "validate-number"=>true, "min_text_length" =>10, "max_text_length" => 10 ];
}
/* config: checkout/options/display_billing_address_on = payment_method */
if (isset($jsLayout['components']['checkout']['children']['steps']['children']['billing-step']['children']
['payment']['children']['payments-list']['children']
)) {
foreach ($jsLayout['components']['checkout']['children']['steps']['children']['billing-step']['children']
['payment']['children']['payments-list']['children'] as $key => $payment) {
/* telephone */
$jsLayout['components']['checkout']['children']['steps']['children']['billing-step']['children']
['payment']['children']['payments-list']['children'][$key]['children']['form-fields']['children']
['telephone']['validation'] = ['required-entry' => true,"validate-number"=>true, "min_text_length" =>10, "max_text_length" => 10 ];
}
}
return $jsLayout;
}
}