Contact Form - Phone Number Field

I'm using this plugin for a client's website, and they want us to add a phone number field to the contact form. I don't see a way to do this in the documentation. Is this a feature of the contact form block, or would this need to be done custom?

If it's something that needs to be done custom, is there documentation on how to do that? I don't know if I could just create my own plugin, append a column onto a table in the database, and add the field to the form via a hook or not.