Reading additional fields in PMPro

I am currently working with PaidMembershipsPro, so I wanted to simply add a field with a phone number using register helper and check if the phone number has already been registered on the website. To do so, I used the hook “pmpro_after_change_membership_level”, and ran into a problem that writing print_r( get_userdata($current_user-ID)) doesn’t show the phone number at all. I guess I have to use the dome special function from PMPro, but I cannot find it anywhere. So how do I do that?