I put my phone in my front pocket. My front pocket can accommodate reasonably thick items like my wallet, but if they're too wide and long, it interferes with my ability to bend at the waist.

It might be local clothing design, but I haven't seen a back pocket so low that you could actually sit on a phone.

The geometry works out that the pocket is halfway from your back and your leg, so when you sit the phone would be at a 45 degree angle.

Half of it would be under compression from your weight in the horizontal direction, and the other would be in tension from the pocket pulling it into the vertical direction.

Not a healthy situation for your comfort, your phone's integrity, and the pocket's half-life, regardless of a phone's dimensions. In fact, a larger surface-area phone would work even more as a lever than a vaguely thicker one.

So I just put the phone in the front pockets.

People put phones in their back pockets?

People sit on their phones?