My Otterbox came with a snap-in "outer case" with a belt clip. I ran two large zip-ties through the belt clip portion and attached it as shown. The standard case's snap-in attachment to the belt-clipped outer case is extremely stout and I've had it stay put through hard hits. I did this as it was the only way for my Bluetooth audio to not drop out. Backpack and pocket didn't cut it. Watching movies not recommended!

(Mirror got rotated 180* and the stem cut down!)
Most commercially-available phone mounts are weaksauce, just waiting to throw your phone under your wheels.