Sadly, though people should know better, we've become so attached to our phones that using them at any time or place just doesn't make us think twice. Short of having ushers take people's phones if they ring during the ceremony, there really isn't a lot you can do to ensure no one uses them. But there are a few things you can do that will help the matter. It's preferred first to use a written announcement, whether it's in the program or on a sign as people enter the sanctuary (like above the guest book or right by the door), because that is less intrusive but still presents the necessary message. If those two options aren't possible, you can have someone - like the officiant - give an announcement before the ceremony starts, but I would try to use one of the two other options.