TLDR: - enabled 2FA from iPhone (9.3.5) - no 2FA pop-up - enabled 2FA from iPad Pro 9.7 (iOS 10) - 2FA pop-up shown I have 2 devices: iPad Pro 9.7 with iOS 10 (upgraded from iOS 9.3.5) iPhone 6S Plus with iOS 9.3.5. I decided to disable Two-Step Verification (2SV) and enable Two-Factor Authentication (2FA). At first, I enabled 2FA from my iPad --> Settings --> iCloud --> [My username] --> Password and Security. It worked just fine and when I login to icloud.com or appleid.apple.com from OTHER devices (MacBook, Windows machine, iPhone, etc.), I got the 2FA pop-up in my iPad, asking if I am allowing login from xxx place. If I click Allow, then the 2FA number will show up. However, this pop-up does NOT show up on my iPhone, only on my iPad. Thinking that maybe 2FA is specific to the device where I enabled 2FA, I did this: Go to appleid.apple.com and disabled 2FA Go to my iPhone 6S Plus (iOS 9.3.5) and enable 2FA from there: Settings --> iCloud --> [MyUsername] --> Password and Security Everything seems fine, but when I tried to login to iCloud.com or appleid.apple.com from OTHER devices (not the iPhone), I did NOT get the pop-up on my iPhone. I can generate the code by going to Settings --> iCloud --> [MyUsername] --> Password and Security --> Get Verification Code, but no pop-up. I rebooted the iPhone, disable 2FA and enable 2FA again from my iPhone but my iPhone NEVER received the 2FA pop-up. The iPad Pro 9.7 (iOS 10) also does NOT receive the 2FA pop-up. However, if I enable 2FA from my iPad, the pop-up ALWAYS show-up. Is this because iOS 9.x.x does NOT support 2FA? If not, can someone help me out? This is very troublesome since this is a security feature and I may ended up getting locked out from my Apple account.