Your cheapest option would be a headset with a microphone. You could then dub the audio if you still wanted to use the phone as sjcs suggested or use a webcam and directly hook up both to a computer to simultaneously record without having to edit later. Cheap mics have to be mounted close to the source to get good quality. A handheld Mic held by the speaker will also work, and be reasonably cheap. Keep cable length in mind when choosing a Mic. You don't want anything that's too short.