When does creating a new account require phone verification?

I've created a system for aging accounts automatically (random status updates/picture uploads/accepting friend requests/etc/etc). The only hurdle right now is creating new accounts. We have some people on staff that see a phone verification requirement once every 10 times when manually creating accounts, so I'm wondering what I might be missing in my code to make this work. My code currently has a 100% phone verification required chance.

Different IP residential private proxy each signup, and still having the same problem; so I know it's related to my code triggering some red flag.

Does anyone have any insight into what metrics Facebook is using to flag a signup as requiring phone verification?