Plus, ringtones are embarrassing.
On every shoot, there’s at least one person whose phone rings. The first time it happens, there’s a little bit of razzing, and then you move on quickly. The second time it happens, it’s trouble. If it’s not you that first time, use that tiny bit of razz-time to make sure your phone won’t go off. Even if you’re sure, check again. The second phone must never be you.
Of course, you can pre-emptively set this up, if you’re the Director, by deliberately leaving your phone on and having someone (such as your AD) ring you. That way, it can break the tension a bit, and give other people a chance to shut their phones off without getting the Hairy Eyeball as a prize.