[saagarjha]

I think you need to provide approval from one of your other devices, or enter your iCloud security code before the sync can occur.

[dgrove]

iCloud security code can come over SMS if your account is configured as such, therefore the above example of a SIM port applies

[tptacek]

No, the iCloud second factor can come over SMS. That's not the same thing as your iCloud password.

[saagarjha]

I'm not 100% sure about this, since it's been a while since I've added a device to my iCloud account, but IIRC the prompt is not the standard 2FA one: it basically asks you to approve the addition from another device, or forces you to use your security code.