Adding a phone number to a signup page can improve conversion over just asking for email. This works in our experience because people are much more likely to answer their phone vs opening an email, and are again much more likely to convert after talking on the phone vs clicking from an email. This does however tend to decrease the landing page conversion rate to form submission, because people are less likely to give their number. So make sure you track whether the dip in form submission is worth the higher conversion rate to purchase.