Thanks to SpOOky who runs the Lost Continuity section) for finding this.
Update from SpOOky: "I just checked it out, and It seems although it sounds like Charlie. It match's exactly how Hurley said it, So I think it may Indeed be Hurley.
The echo makes it sound like Charlie, but I would expect some slight
variation in its timeing. However it seems to perfectly match Hurley."
It happens during the flash desmond has when charlie says "I guess its
supposed to happen without me"
It has Charlie shouting "Hey can you hear me"
Source: SpOOky