I reckon

mom: ...and once a week the iceman....
john lennon: cometh?
mom: ....came and brought a big block of ice this big (some hand gestures) and put it in the top of the icebox.
john lennon: did you get that?
rob: ha yeah.
mom: what?
john lennon: the iceman cometh. you left a big pause after iceman.
rob: hey.. was I Icebox or was dave?
john lennon: Dave was. You were Bigtime
rob: oh yeah. ha ha.. we should record this conversation for posterity.