Can we add API validation requests to form?

Is there a way to make an API request in the middle of a form for validation? Here's the use case I am curious about - I have a form where I ask for someone's phone number. I want to send an API request to our system to check if the phone number is already in our database. If it is, I want to send an error message within the form. If not, I want to continue asking questions. Is this possible? Thanks

This is only possible if you use a classic layout form with your custom code modifications.

The custom code cannot be directly added in our builder, but you can get a form source code, then host it on your own webpage with your code modifications.