I've been party to the occasional conference call and haven't been really impressed with the hardware used. The voice quality wasn't always what I expected. Sometimes it was tinny, sometimes distorted, sometimes far away depending on where the speaker was positioned on the other end of the call.
Makes me wish we were using Polycom VTX 1000 Conference Phones back then. These models are designed to be able to pick up speakers up to 20 feet away from the microphones and offer full-spectrum sound. This is kind of neat - because full-spectrum sound refers to the hearing frequency range of us humans. The Polycom VTX 1000 can reproduce frequencies from the bottom to the top of the human hearing range, resulting in fuller, richer sound reproduction. Polycom says it's like having the other person in the room with you.