When to ask for phone verification?

This is related to a food ordering/takeout mobile app. Currently, users create an account immediately in order to access the app, and it is required that they verify their phone # to prevent duplicate accounts. What is more likely to retain users/will cause less abandonment:

• Asking them to verify their phone # during the account creation/sign up process?
• Asking them to verify their phone # when they attempt to checkout with an order for the first time?