To configure a model-driven app in order to provide a single screen for the sales department users to enter the customer data, you should modify the site map of the app. A site map is a hierarchical representation of the different areas and functionality of the app, and it can be modified to create a new screen or view that combines the necessary fields for the customer data entry.
Once the site map is modified, you can add the necessary fields (first name, last name, and phone number) to the new screen or view, and make it accessible to the sales department users.
Reference:
https://docs.microsoft.com/en-us/power-platform/admin/model-driven-apps-overview
https://docs.microsoft.com/en-us/power-platform/admin/modify-site-map