You can customise some of the fields available to the sponsor from the Portal Page Customization > Create Accounts > Create Accounts for Known Guests menu. On the right-hand side, click the Settings tab.
Note that you can only remove the Email address field if you are not using the email address in your Guest Username Policy (Work Centers > Guest Access > Settings.
The Mobile number field can also not be unticked in ISE 3.0 due to bug CSCvx37467