There is a difference between "Indirect Signature Required" and "Direct Signature Required."
"Indirect Signature" does allow you to leave a signed note for the delivery to be left where you indicate. However, "Direct Signature" is just what it says, an adult (over the age of 18-years-old) has to be there to physically sign for the package.
As far as I know, all of the iPhones are being sent requiring a "Direct Signature." That means that someone has to be there to sign for it. I know that there are always exceptions, especially if you know your delivery person, but technically someone will have to sign for the package or it won't be delivered.
P.S. I've tried and in my experience, I haven't been able to get any door tags.