I watched the episode again. At 34 minutes and 29 seconds into the episode, before Columbo and Dr. Fleming go to the hospital, Joan Hudson indeed calls. Dr. Fleming makes it seem to Columbo like the caller is a patient. Thanks to your hints, guys, I now see that a patient calling Dr. Fleming's home number is indeed suspicious. And I now also connect Columbo's deliberate visit at 2 o'clock on Friday, to find out the name of that home-caller. Thanks! I can even add: Earlier, Dr. Fleming explicitly asked Joan not to call/visit Dr. Fleming for a few days. When Joan does call Dr. Fleming and apologizes for calling, Joan actually apologizes for ultimately exposing Dr. Fleming as the murderer.