Somewhere on this forum, someone else mentioned that the signal strength was low when the phone was unactivated. But the signal strength was greatly improved once the phone was activated. So there is some factor going on there.
Since you bought the phone at return-friendly Target, you could always activate and check everything out. Then if you want to transfer from your other phone, you will need a new sim for the next phone number, but that is not a problem.
Speaking of sims, check the package and see if there is a P4 associated with the upc/model number. Or a sim number beginning with 890141. Those indicate AT&T.