Cell Phone vs Conversation

It's odd that hearing someone talk into a cell phone is annoying but hearing two people having a conversation usually doesn't bother anyone. I suppose you could argue that one-sided conversations are more grating because they're loud then quiet and tough to get used to, but I'd argue that they're more annoying because eavesdropping is impossible. A two-sided conversation eventually becomes the hum or your refrigerator or fish tank -- you simply tune it out because you're uninterested. A cell phone conversation gets more annoying as it goes on because you can't figure out what you're trying to tune out.