marketpress - is there a simple way to make the phone number required on the shipping form?

Is there a simple way to make the phone number required on the shipping form? (basically, we are not shipping, just customer pickup, but there appears to be no option for that, so I have set shipping to flat rate of $0 in order to get the contact(shipping) form to come up in the checkout process, but we want the phone number field to be required.)