The opening scene of the first episode of Get Smart in 1965 was a scene where Max's shoe phone rang in a crowded opera house. Not only was it a hilarious opening scene, it was eerily prophetic.
Gemini VIII was launched on March 16, 1966, so it is quite likely that the "Emergency Communication earphone" was a joke that would almost certainly have been funny to anyone who was a fan of the Get Smart show at the time.
Phone starts ringing...
Max excuses himself...
Max retreats to a closet and answers the phone.