Apple's first foldable phone, the iPhone Duo, went on sale Friday, Oct. 23, and many stores sold out of their limited walk-in inventory within hours, Bloomberg and the New York Post reported.

Why it matters: The Cupertino company's Duo is the biggest design change in the iPhone's history and one of the most expensive mainstream phones ever, and the iPhone line brings in roughly half of Apple's revenue.

Launch day

U.S. Apple stores received roughly 50 to 200-plus Duo units each, Bloomberg reported. A Los Angeles store had none left for people arriving after about 6:30 a.m. In New York, the Fifth Avenue store had a line in the early morning hours and had run out of walk-in stock by mid-morning, according to social media posts cited by the Post.

The phone starts at $1,999 for 256 GB, with models up to $3,199 for 2 TB. New online orders are being offered delivery at the end of January, though some shoppers have seen estimates move up to November or December, Bloomberg reported.

New CEO John Ternus greeted shoppers at an Austin store, Bloomberg reported, citing CBS.

Why supply is tight

The Duo is hard to make because of its hinge and new display, and it is built in a limited number of facilities in China, according to Bloomberg.

What's next

Apple reports quarterly earnings Nov. 2. Those results will not include Duo sales, because the quarter ended around late September, Bloomberg reported.

Sources: Bloomberg via The Hindu BusinessLine; New York Post.