Supposedly (not sure if this is from the "new" or old style screen. I know that from my mobile device I didn't see the expected links.) the following would be performed:

To require a phone number of a customer who checks out with Paypal:
Sign in to your PayPal account.
Click the Profile icon at the top right.
Click Profile and settings.
Click My Selling Tools or My Selling Preferences under My Profile on the left.
Note: This button might differ depending on your PayPal account.
Click Update next to Website preferences.
Scroll down to the Contact Telephone Number section.
Select On (Required Field).
Click Save.