AT&T Cell Phones Being Shipped to Vacant Address by Scammer

There have been two incidents of cell phones being delivered to my apartment complex's vacant unit, where the addressee doesn't live there. The boxes sit on the doorstep for while, with nobody coming by to pick them up. I have tried to return the packages to UPS, both at their store, and via the driver picking it up, but UPS says it is my problem not theirs, and they refuse to accept the package even though there is a tracking number on it. 

I haven't opened the boxes, but am presuming since they are from AT&T they contain expensive cell phones, maybe even $950 iPhones. The landlords says it is not their responsibility and I should just leave the package in the gutter for the trash collection to pick up. 

It's considered Grand Theft to steal anything > $950 in California, and I am now, according to the UPS customer service representative legally responsible for returning such items, as I am accessory to the fact if there was theft involved. Seeing as how this is probably some scammer, I'm wondering how to deal with this? Local AT&T store won't accept the package, so am at a loss as to what to do.