Filk is an obscure Seinfeld reference. Kramer's phone number, at least in "The Pool Guy," is 555-FILK, which is one number off from MovieFone (555-FILM). When he gets constant wrong number calls from people looking for the movie service and dialing him instead, he pretends to be the touch tone movie information service. However, he decides to stop doing this when the MovieFone guy threatens to break down his door. (Supposedly. Being as the episode actually ends then, we can't be quite sure, but he certainly has a door in the next episode. This could lead to an argument about continuity but that's a whole 'nother node.)