Where did FB get this number from?

My husband's FB profile had NO phone number established. He had recently removed it due to privacy concerns. Soon after deleting his cell # without replacing it, FB prompted a profile update upon Safari (iOS 9) log-in.

"Help keep your account secure so you can reset your password if you ever need to, find friends, get SMS updates and more"

::insert random number pre-filled here::

Where in the world did they get the random number?

TIA :-)