Haha I ALWAYS have this problem. Normally I just leave it in my bag but if I do take it riding, I tuck it into my jodhpurs at the front. It can be a little uncomfy but it works!!
I usually don’t ride with mine, but if I do I put it in my horse holster(which is pouch that holds your phone and wraps around your arm or leg). Also, they sell ones that clip on to the back of your belt which I think are much more convenient and I want to try! Other than that, I just leave my phone on the arena fence
I usually leave mine in my tack trunk, or in my boot. Since you can’t put it in your boot then I would suggest getting a holder thingy... lol what a good description 😂. But anyways my trainer has one that straps around her leg. Not sure where she got it but I’m sure it’s just like what others are suggesting
There are some wallet - pocket that you put on your belt. Equestrian prep (youtube) uses them
I have a special pouch that wraps around my arm to hold my phone when I ride, kinda like the one @amy.sandersrider
showed you. I got mine at Dover, for about $20. The only difference between the runner one and my Dover one is that my Dover one has a horse on it😂. Mine closes magnetically, and has a velcro strap that wraps around my arm to stay pretty secure. My only complaint is that I have to almost cut the circulation off in my arm to make it stay up, other than that, it works well!
You can put your phone on your boots or even your bra but usually it is moving once you start to trot.
I would recommend you to buy a special outfit for runners that you can put on your arms that are specific for phones such as this one:
Hope I could help :)