I am flying as FO on the B737 NG and during a night low visibility approach we always heard the sound a mobile phone makes when it tries to find a net, you know that didididadada in our phones. It was really harassing, probably some kind of passenger trying to contact someone. Shortly before the self confidence test our autopilots disengaged. We made a go-around, made an announcment to please switch off all electronic equipment and gave it another try. It went all straight - like in the books.
Well in the end we were not shure what made the autopilots disengage, and I really can't tell. But I think maybe the cellular phone had something to do with it. But that is my personal opinion - nothing official.