An addon for WPForms plugin, allow to force and valid french format phone number for the field with the class: "wp-forms-check-phone-number"

wpforms-pattern-french-phone.php

<?php
/**
 * Plugin Name: WPforms - French Phone Validation
 * Plugin URI: https://beapi.fr
 * Description: Add JS and PHP check for validate french number, supported formats : +33123456789 and 0123456789
 * Version: 0.2
 * Author: BE API
 * Author URI: https://beapi.fr
 */

add_action( 'wp_footer', function () {
	?>
	<script>
	(function($) {
		var WPforms_FrenchPhone_Validation = {
			init: function() {
				$(document).on('wpformsReady', WPforms_FrenchPhone_Validation.customRules)
			},
			customRules: function() {
				if (typeof $.fn.validate !== 'undefined') {
					$.validator.addMethod(
						'regex',
						function(value, element, regexp) {
							if (regexp.constructor != RegExp)
								regexp = new RegExp(regexp)
							else if (regexp.global)
								regexp.lastIndex = 0
							return this.optional(element) || regexp.test(value)
						}, 'Votre numéro de téléphone ne semble pas valide'
					)
				}

				if ($('.wp-forms-check-phone-number input').length) {
					$('.wp-forms-check-phone-number input').rules('add', {
						'regex': /^(\+33|0)[0-9]{9}$/
					})
				}
			},
		}

		WPforms_FrenchPhone_Validation.init()
	})(jQuery);
	</script>
	<?php
} );

/**
 * Add PHP validation for custom rule
 *
 * @param int $field_id
 * @param array $field_submit
 * @param array $form_data
 *
 * @since 1.0.0
 */
function wpforms_process_validate_text_french_phone( $field_id, $field_value, $form_data ) {
	$form_id = $form_data['id'];
	$fields  = $form_data['fields'];

	if ( ! empty( $fields[ $field_id ]['css'] ) ) {
		$classes = explode( ' ', $fields[ $field_id ]['css'] );

		if ( in_array( 'wp-forms-check-phone-number', $classes ) ) {
			if ( ! preg_match( "/^(\+33|0)[0-9]{9}$/i", $field_value ) ) {
				wpforms()->process->errors[ $form_id ][ $field_id ] = 'Votre numéro de téléphone ne semble pas valide';
				return;
			}
		}
	}
}

add_action( 'wpforms_process_validate_text', 'wpforms_process_validate_text_french_phone', 10, 3 );

I faced a similar problem and ended up building a free WordPress plugin that handles this without touching any code. It supports minimum/maximum length, blocks repetitive digits like 1111111, has a blacklist, country code filtering, IP throttling, and validation logs. Works with all WPForms phone fields automatically. Might be useful if you need a more universal solution:
https://wordpress.org/plugins/phone-validator-for-wpforms/
https://github.com/Ilyntiy/phone-validator-for-wpforms

Great !