Phone Storage Water Bottle
If your workout allows for carrying or keeping a water bottle nearby, I'm sure it also allows for a separate place to stash your phone and other personal items. But if you can combine the two, and keep everything together in a single BPA-free container - which also creates the trippy optical illusion of making it look like your phone is living in a bottle of water - why not?
The Primula wet water bottle / dry storage compartment has a 16-ounce water reservoir on one side and a waterproof stash slot for standard-size phones, cards, cash, and IDs on the other. The bottle's base also has a headphone access port, and the lid on top is leak-proof so your stuff outside the waterproof storage container doesn't get soaked either.