How can I validate the phone number by having them entering it twice just like with email confirmation?

Please how can I validate the phone number by having them entering it twice just like with email confirmation.Pierre 

The closest we have similar to the email validation is the Field Confirmation widget:

But please note that the "Field Confirmation" widget is not limited to just numeric validation. It will match any text be it numeric / alphanumeric or alphabets.

So the best validation for a phone number field would be to use an Input masking feature on a Short text entry field - https://www.jotform.com/help/381-Getting-Started-with-the-Basics-of-Input-Masking 

You may also check out the Masked Input widget - https://www.jotform.com/widgets/masked-input 

Do review and let us know which one would work for you.