I know Phoenix did backups when he was in The Snax, but I've never seen or heard him or any of the LP members besides Mike and Chester sing. Although Phoenix did have a mic in front of him in the Its Goin Down vid
well i have, which is why i say that. he even raps a little bit in the bonus track from Run Joseph Run!
The only possible thing I can think of is samples. Maybe Joe has some samples of the other guys talking. Like High Voltage (remix). That was slated for a Hybrid Theory release originally. I'm thinking that the phone call was Phoenix or Brad. Also, just some of the samples Joe used for scratching may have had vocals in them. Who knows.