It's unlikely that the shipment is sent "Signature Required", that's an extra-cost service and would not be a good value for Sprint/Apple.
However that does not mean that they will simply leave the phone on your front porch. They might do so (happens with me all the time - but it's a small town, house set well back from the street, covered porch, not visible from the street) but you can't count on it.
The UPS driver will not leave a package unattended if he feels that the location is not safe to do so. You might leave a note with instructions, if they are reasonable (i.e. don't cost the driver time) most will be happy to accommodate. Notes like "Take it 3 doors down and give to Myrna ONLY IN PERSON and if she's not home please take around to the back and hide it under the Chrysanthemum bush" will definitely be ignored. Notes like "Please leave packages in the Rubbermaid bin next to the door" are much more likely to be honored.