In Lost, what was the name of the episode about Desmond and Penny’s phone call?

In Lost, the episode about Desmond and Penny’s phone call is “The Constant.”

“The Constant” is the fifth episode of season 4 and follows Desmond Hume after a freighter experiment causes his consciousness to shift between 1996 and 2004. Daniel Faraday helps him understand that he needs a stable person, or constant, to reconnect his memories across time.

Desmond ultimately contacts Penny Widmore by telephone. Their conversation gives him the emotional anchor needed to return safely to the present. The episode combines science-fiction ideas about time displacement with the continuing story of Desmond and Penny’s relationship.

The episode is often remembered as one of Lost’s most acclaimed installments. It is separate from “Flashes Before Your Eyes,” which also focuses on Desmond’s unusual experiences but uses a different narrative structure and story premise.