Validate phone field for 9 digits

validate_phone_field_for_9_digits.php

add_action( 'woocommerce_checkout_process', 'validate_billing_phone_9_digits' );

function validate_billing_phone_9_digits() {
    $phone = isset( $_POST['billing_phone'] ) ? sanitize_text_field( $_POST['billing_phone'] ) : '';

    // Remove all spaces and non-digit characters before checking
    $phone_digits = preg_replace( '/\D/', '', $phone );

    if ( strlen( $phone_digits ) !== 9 ) {
        wc_add_notice(
            __( 'Please enter a valid 9-digit phone number.', 'woocommerce' ),
            'error'
        );
    }
}