That’s already a feature that the Calendar extension gets from the main/free plugin, since the main plugin generates the HTML for the sign up form, even on the calendar.
However, if I remember correctly, the user needs to sign up for at least one thing first, filling out those fields, while logged in, and then that info is saved to the user meta database and used to auto-fill the fields the next time they sign up for something. The basic WordPress registration is only a user name, password, and email. So, there is no first name, last name, or phone stored in the user meta unless you allow them to edit their profile and add that info (and phone is not a default field either). At the minimum, if they are logged in, it should populate the email field, even if they have never signed up for anything before. Once they sign up for something, then the first name and last name are stored in the user meta database and can be auto filled for future signups.
I just double checked on my dev site, and auto fill is still working just fine for my account on the calendar sign up form.