Thanks all. For the record, I have an Android phone, and I went into the "Bluetooth" settings, found the device ("Sync"), and unchecked "Phone" like Rebecca said. I'm not sure about how to do it with an iPhone, but it's probably similar.
And akirby, I prefer not to use speakerphone, it's just a pet peeve. I use a headset when I'm on the phone while driving, and I find it makes it much easier to understand, and be understood by, the people on the other end.