The message left on the answering machine at the opening credits changes from episode to episode, usually as some kind of gag. Occasionally it dealt with some part of the forthcoming story. Frequently the voice was of supporting actors on the show, either as themselves or as a completely unrelated character.
The credits for guest stars, writers and other upper-level crew did not always appear right away, sometimes appearing as late as ten minutes into the episode.
If you look closely at the beginning credits/ sequence you can see near the end what appears to be a silhouette of a female streaker for only about a fraction of a second.