Moriarty kidnaps Sherlock and forces him to call John and initiate phone sex with him. No established relationship beforehand. Moriarty is shocked and delighted when John, after a minute of stuttering disbelief, goes along with it. Afterwards, Moriarty lets Sherlock go.

And I mean NON-established. Not even flirting or longing looks.

Bonus for aftermath! Maybe John waits for Sherlock to return so they can talk about what happened... Well, what John believes happened.