@mark It was done via #simhijacking, also known as the #portoutscam: https://motherboard.vice.com/en_us/article/vbqax3/hackers-sim-swapping-steal-phone-numbers-instagram-bitcoin
It happened while I was in the hospital. Later found it listed on an auction site, where it sold for a lot of money.
Fortunately, I was an early user of #Instagram (when it was called #Burbn), and CEO Kevin Systrom responded to my plaintive tweets. He got it back for me.
Of course, the buyer and his friends were very upset and harassed me and my family for a while. But it's calmed down since!