You can use [%payer_phone%] for the buyer's phone number and [%shipping_phone%] if they provide a different phone number for their shipping address.
If you want to make a phone number required, you can enable a setting for that in Seller Admin > Payment Preferences, but note that would not apply to checkouts on PayPal's site, which would use a setting for that in your PayPal account Profile > My Selling Tools > Website Preferences.