There is a paypal configuration setting you have to change on the actual paypal site to make it pass back the phone number.
Are you asking for the phone number on your site before checkout? If so, then you can always use that one instead by removing the line that gets the value from paypal on the pp_confirm.php page.
To change the setting on paypal you have to turn on "Require a phone number to be entered":
You can require a phone number to be entered for guest checkout users. This is available via Profile > My selling tools > Payment receiving preferences > Contact > phone number > Require a phone number to be entered.