The built-in forms of Wishloop support, by default, first name, last name and email fields only.

However, if you want to integrate your custom opt-in forms with a phone number field for example, or any other custom field, there are actually two ways of integrating custom opt-in forms :

If you want to preserve the native design/style of your form, use the widget located under Misc > HTML Code in the left panel of the Campaign Builder. Drag it to the canvas and go to its settings and simply copy and paste your form’s HTML Code.
If you want to use Wishloop's built-in design options, use the widget located at Forms > Custom Opt-in Form. To set up your custom form to use your custom HTML code, check the following link: How to integrate my autoresponder? [HTML Code option]

You're free to try both widgets and decide which one works best for you.

How to integrate my custom opt-in form?