This study controlled for loudness and ringtones, but commuters still found mobile phones more annoying. I suspect it’s partly due to prejudice, but an interesting hypothesis is raised:
Unfortunately, Monk and his colleagues don’t provide the final answer; more research is called for. But the problem seems to be that people pay more attention when they hear only half a conversation. It’s apparently easier to tune out the continuous drone of a complete conversation, in which two people take turns speaking, than it is to ignore a person speaking and falling silent in turns.