HubSpot does validate email addresses because there are backend records on the email addresses that can be checked online to verify if an email is valid or not. And so this is something that the forms tool does. If an email is not valid, it will not allow the form to be submitted using that email.
But when it comes to a phone number, we can choose to have the country code chosen. However, this will not validate if the phone number is correct or not according to the country. The only validation rules that you can add in a form phone number field are the minimum number of digits and the maximum number of digits that the visitor can type.
It would be really helpful if we could have a similar system to the email validation for phone numbers. I believe this is very much needed. 🙂