I got a SIM-Free 64GB Space Gray 6s+ today at noon, paid in full. Walked over to Verizon store to ask for a SIM (my old device had a micro SIM , not a nano SIM). Verizon checked me in, I waited 15 minutes while looking at cases, decided on a sleek Otterbox, and the girl from Verizon installed the SIM and switched over my Unlimited Data Plan to the new phone. Took 5 minutes for the new phone to successfully make a call, and have had no issue since.
FYI, my 6S+ has a Samsung A9
EDIT: the sim from verizon was no cost.