Samantha's Witchcraft Blows a Fuse - S8-E25
Other mistake: When the police pull up outside the Stephens' home, it is a set shot with fake background. The "road" doesn't match the outside of Stephens' neighborhood shown throughout the series with the Kravitz house across the street (and not a pine tree forest as shown here).
Answer: Telstar was an early communications satellite system. Darrin, in a particularly sexist manner reflective of this era, is saying that telling a secret to a woman means it is guaranteed to be "broadcasted" to multiple people.
raywest ★