Wirelessinfo knows what they're talking about when it comes to cellphone calls. They're the Mythbusters of cellphones, pulling out all sorts of scientific equipment to determine whether call quality is good—not just doing a "oh, this sounds good" test. Their results for the iPhone? Best they've seen.
It scores higher on the frequency response test compared to the BlackBerry 8800, Helio Ocean, LG Prada, Nokia N95 and the Treo 750. The audio quality is great as well, as is the send frequency response. The iPhone did have a bit of "side tone", which is the amount that you hear yourself in a conversation.