With the use of the McDSP Futzbox or with free plugins (ReaEQ mainly) included with REAPER, you can create a distorted megaphone vocal effect. Basically what you are doing to make any audio sound like it is coming from a megaphone (or telephone) is emphasizing the middle frequencies and pretty much deleting the low and high frequencies. You can just create a narrow bandwidth in the middle of the frequency spectrum, and filter out the rest of the frequencies with the EQ plugin. See picture on the left. Watch the video from the Reaper Blog for more information.