That happened to me once, and I was mortified! It was a new phone, and I thought that I had it on silent. I did not know what to do. I tried to silence it. I did not want to look like I was texting someone. It truly distracted my attention during mass. I did finally figure out how to turn it off. Lesson learned - do not take new phone into mass until you really know how to operate it. It was a long time before I even trusted myself that it was actually on silent, so I left it in the car. However, when the weather was really cold I was afraid of ruining the phone. I would have left phone at home, but roads were iffy, and I wanted to have on me if I had car problems. Now, I know how to operate my phone. And, now that weather is nice, I will leave in car.
It made me more understanding of when someone phone goes off. Most people do not have it on intentionally. Once, a lector was preparing before mass, and had a phone call from his wife. It was kind of comical, as you can hear him yelling that he was at church. He did not know how to turn phone on silent because he got another call. However, they were before mass actually started. Phones can be quite embarrassing when they go off in places they shouldn’t.
We now have a sign on the door to shut off cell phones. And, yet, we still hear someone’s phone going off.