there is not a way to access a customer's receipt email address or phone number as those are for receipts and not for contacting the customer. you can get that information at checkout that you can access by enabling the customer directory feature of Square. any information entered into Square Customer Directory is accessible to you.
https://squareup.com/help/us/en/article/6217-get-started-with-your-customer-directory
https://squareup.com/help/us/en/article/5532-manage-your-customer-directory-from-the-square-app
