I turn off my phone before entering church and at the cinemas. If I am at the doctor's surgery, I will ask the attending doctor or nurse if it's non-negotiable.
Like Finy, my phone hardly ever rings anyway, I'd still turn it off before going in to the movies, to see a play or when I very occasionally go to the opera.
I have had a couple of x-rays done, but I wasn't asked to turn off my phone beforehand, so I didn't think of doing that myself of cause!