by Jim » Mon Sep 19, 2011 12:11 pm
The billing information is already on the form, whether they check or uncheck the check box "Check here if billing and shipping are the same", and that information already contains the phone number for the shopper which by default is a required field.
Are you going to be asking for the phone number of the credit card company? If that is the case that would really be unusual. I have never been asked for a credit card company phone number in the 35+ years I have been using credit cards. The number listed on the back of the card is for the cardholder to use for service not for merchants. Your payment gateway should be able to validate if the address given matches the address on file for the card.
Custom fields can be added to the cart checkout screen in the Pro version of ShopSite (Manager and Starter don't have the feature). Look at the section "Custom Checkout Fields" found on Commerce Setup > Order System > Checkout.