Takayuki Miyoshi on "[Plugin: Contact Form 7] Tel number field error"

The default validation function for tel number only accept number and - and +. To add ( and ) to it, add this code to your theme's functions.php:

add_filter( 'wpcf7_is_tel', 'your_custom_wpcf7_is_tel', 10, 2 );

function your_custom_wpcf7_is_tel( $result, $tel ) {
	$result = preg_match( '/^[0-9()+-]*$/', $tel );
	return $result;
}