An iPhone 6's dimensions are 5.44 x 2.64 x 0.27 inches. If you laid them flat on a football field, orienting them the same direction as the field, you could fit roughly 661 between in columns between endzones and 727 in rows between sidelines. That would give you a single "layer" of 480547 phones. To reach a billion, you'd need 2080 layers. At 0.27 inches thick, that would rise 561 inches into the air, or nearly 47 feet. That's a 4 story building plus an NBA player.

Hopefully that math checks out.