Creating Activities - Forms

Hi,

I have added some custom fields to the Phone Call activity, and need to surface these in the Create Form.

There are 2 options I have tried 'Main Form', or add a new 'Quick View Form':

If I cannot create a Phone call activity using the Main form and am stuck with using a quick form, how can I set the default to be my own custom form, because we are unable to edit the default, managed form.

Thanks,
LJ