For most people, a Christmas Eve phone call with the first lady of the United States is an unexpected surprise. But Austin was holding out for someone else.

“I want to hear Santa talk,” said Austin, one of a handful of kids who called the NORAD Tracks Santa program on Saturday and found Michelle Obama on the other end of the line.

Santa couldn’t get to the phone, the first lady patiently explained, “because he’s delivering all the gifts” — 3 million as of late morning in Honolulu, where President Barack Obama and his family are on vacation.

There was a bittersweet note this year as the first lady carried out her annual Christmas ritual for the final time,…