I think the risk of having a phone fall out of a jersey pocket is going to heavily depend on its size (e.g. snug fit or loose fit), as well as how bumpy the ride is. I keep my phone in a Rapha large rainproof case, which then fits snugly into the pocket (and on some tighter jerseys, takes a little bit of effort to push in) so that makes it very unlikely to fly out. It probably also helps that I keep my keys, cards, mask and other personal/essential crap in there, giving the case more "fit".