The telephone Barney is using is actually a telephone repairman's test set. It doesn't have a ringer. Besides that, the phone line would be dead in an unoccupied apartment.
During the stakeout, when Barney is helping Fish to the washroom, Mrs. Stavochek enters the apartment asking to show the apartment to the prospective tenants she has with her (16m 52s). In the foreground, bottom-center of the frame one can see the hair of a crew member who had just moved from the right. (Crew member is actor Eddie Barth who played Detective DeLuca.)