When participants sign up for their spots, SignUp.com automatically asks for a phone number. This gives the Organizer a way to contact the participant (parent, volunteer, etc.) in case the email account becomes unavailable.
The phone number field is a text field. If you don't want to collect participant phone numbers, please ask your participants to put " * " or "n/a" in the phone field to proceed with their SignUp. You can put this message in the invitation text or in the Welcome Message.