How to validate the phone number in conatct 7 form

The CF7 [tel] tag implements the new HTML5 tel input type which has no real validation – see https://www.wufoo.com/html5/types/2-tel.html.

See Custom Validation.?

Hi Buzztone,
Thanx for quick reply .
i had tried the code that you mentioned link but its not working for me.
what i want that tel (phone number ) is not less than 10 digit .
i had place that code in theme => function.php file .