I bought a cheap Android "dumbphone" specifically for going to the gym. I was constantly getting distracted responding to messages / browsing the internet while working out, so on this phone I only have my workout tracker and Spotify installed. I disabled the Chrome browser though an app restriction tool. The phone has a cheap data-only SIM. My regular phone stays in the car.

I noticed this made a huge difference, I'm much more focused on training and my gym sessions finish a lot faster.