I'd not do anything until they ring back
then the whole time they spoke I'd pass wind through the mouth piece

though obviously I cannot fart on cue like that nor would I like to use the mouth piece afterwards. So I would prepare a tape recording earlier.
( glued eel /questions/questionsyoudliketoask/post1648081, Wed 10 Jun 2009, 18:29, archived)

I can.

( Moon Girl Technologies horrendous beanbag, Wed 10 Jun 2009, 18:33, archived)