Facebook doesn't allow websites/applications to download the user's phone number. We are allowed to access many fields about a user, including email, but phone is not one. Since Joomla requires an email address for each user, if they don't have one set in Facebook, then they will need to go through the mini-registration process.
There's no way around this that we've been able to think of. The same issue happens with Twitter which does not let websites retrieve the user's email at all.
I hope that helps explain, but if you have any other questions, feedback, or suggestions, please let us know.
Thanks,
Alex