A clever Redditor has come up with a way to escape trouble if you are one of those folks in the habit of forgetting to turn off their ringer.
As a college student and waiter who forgets to silence his phone, this has saved me from death glares from professors and customers when my phone goes off. Works great for movie theaters and office meetings. While yes, people will notice the sound, they will not necessarily know what it was or where it came from.
Other suggestions have already come up. Things such as sneezes, quarters dropping, etc. In other words, just natural, everyday sounds that wouldn’t me mistaken for a phone ringer. Nice.