I think phone boxes will become rarer. We are not at that stage yet but the day will come when they are viewed in the same way as police boxes. 50 years ago when Doctor Who started a police box was a common sight but of course not any more. The irony is that when that happens I would suddenly become interested in finding caches placed by/in them!
Another irony that occurred to me is that telephone boxes (and the caches placed in them!) are being killed off by mobile phones. The very things that I think people use to find them!