My phone is in a Lifeproof case and I attach it to my leg just below the knee using an armband around my leg. I only ride around my own property and up the road, so I don’t have to worry about a lot of brush or such getting caught on it, but that might be a problem if I was out in rougher areas. I want the phone with me if I fall, and don’t want it on my waist/body where it might cause more damage (to me!) if I fall on it.
I rarely answer or text if I’m riding, but like to have it in case of an emergency, or to take pictures if something interesting shows up (like the 4 bald eagles flying over head this fall). It was also good to have when my jackass warmblood bucked me off last month and I needed to call for help. That’s why I’d rather have it on me than attached to the saddle.