Ah, the iPhone. Roughly a third of American adults use them. In fact, my analytics suggest there's a better than even chance you're reading this article on one right now.

As we reach the 10-year anniversary of the day it first went on sale--that would be June 28, 2007, in the United States--there are fascinating stories to share about its development that stayed secret for years afterward. Here are the 25 biggest, best, and most entertaining (former) secrets about the iPhone.

(Among the resources this article is based on: Lev Grossman's New York Times article about Brian Merchant's new book, The One Device; Fred Vogelstein's article in The New York Times Magazine; and a recent piece by Tripp Mickle in The Wall Street Journal.)

1. The first iPhone call was ignored and went to voice mail.

How prescient, right? Even the development team was like, "C'mon, man. Send me a text instead!" The intended recipient was Andy Grignon, an iPhone engineer, who told Merchant:

Instead of being this awesome Alexander Graham Bell moment, it was just like, "Yeah, ... go to voice mail." I think it's very apropos, given where we are now.

2. There was almost an iPhone Mini.

Jon Rubinstein, who was Apple's top hardware executive back then, said:

I was pushing to do two sizes -- to have a regular iPhone and an iPhone mini like we had with the iPod. I thought one could be a smartphone and one could be a dumber phone. But we never got any traction.

3. They built at least five prototypes that never saw daylight.

A source said Apple built six working prototypes before settling on the one you and I know (and its descendants). One of these reportedly was "based on the original iPod's click wheel, backlit in blue and orange."

4. Part of the design came from a Tom Cruise movie.

Minority Report, specifically. You can read more about that feature here. (h/t Zach Ezer, Gizmodo).

5. The prototype couldn't recognize dark hair.

There were immense technical hurdles throughout -- even right up to delivery. As an example, Merchant talks about how the iPhone's "infrared proximity sensor," the thing that shuts off the screen when the phone is near your head, "wouldn't recognize dark hair."

6. Jobs wanted a Back button, in addition to "home."

"[It] came down to a trust issue, that people could trust the device to do what they wanted it to do," designer Imran Chaudhri told Merchant. "Part of the problem with other phones was the features were buried in menus; they were too complex."

7. It was all top, top, top, top, top secret.

No surprise, but this was extreme. Interviewing to work on the project (code-named Purple) meant you had to sign an NDA -- and then sign another NDA saying you'd signed the first NDA.

Perspective: Have you ever read any leaked internal Apple emails? Probably not, but thanks to Chelsea Manning, millions have seen top-secret U.S. military and diplomatic reports.

8. They hid the fact that it was a phone from vendors ...

Apple employees wanted some vendors to think they were working on an advanced iPod. They also impersonated employees of other companies when they traveled.

"You didn't want the receptionist ... to see all the badges" with Apple's name on them, Grignon said in Merchant's book. Another engineer added, "We actually had fake schematics and fake industrial designs."

9. ... but also because Jobs just loved keeping stuff close.

Grignon explains:

Steve loved this stuff. It was a big "[expletive] you." ... Everyone knows who the rock stars are in a company, and when you start to see them all slowly get plucked out of your area and put in a big room behind glass doors that you don't have access to, it feels bad.

10. They designed a special screwdriver to make it hard to take apart.

You probably knew this already--but did you know that the proprietary screwdriver has a name? It's called the Pentalobe.

11. All that secrecy led to at least one divorce.

At least, that's what an engineer or designer told Merchant. The development lab became a "soup of misery," Grignon said. It was nicknamed "the Purple Dorm," because people worked there round the clock, through weekends, holidays, vacations, honeymoons. They ate there. They slept there. It smelled bad.

12. The development cost was staggering ...

Vogelstein reports that "one senior executive believes that more than $150 million was spent creating the first iPhone."

Since then, it's led to $740 billion in revenue, so, not a bad investment. The iPhone now accounts for two-thirds of Apple's revenue.

13. ... and $1 million alone went for the domain name iphone.com.

All it does now is forward to Apple's website.

14. Apple today is six times larger than it was in 2007.

Not a secret, but a heck of a story. That's counting employees -- head count is now 6X what it was a decade ago. In terms of market cap, it's more like 8.5X; Apple has gone from about $94 billion in 2007 to just under $800 billion.

15. They've sold 1.2 billion iPhones in 10 years.

Also not exactly a secret, true, but let's put it in perspective. In 10 years, Apple has sold about three times as many iPhones as Ford has sold cars in more than 100 years. Also, 1.2 billion iPhones compares with about 200 million Sony Walkmans (over nearly 30 years) and one billion Barbie dolls (in almost 60 years).

16. They gave an iPhone to almost every employee.

This was the hottest product, and so on the official launch day Jobs announced that all full-time Apple employees (and part timers who had been on board for at least a year) would get free iPhones--delivered a month later.

17. An early user got a 300-page phone bill.

If users didn't check a box, AT&T started out by detailing every email and text sent using the iPhone. This resulted in massive-length phone bills, including a 300-page bill received by 23-year-old Justine Ezarik. She recorded a video about it that went viral (and earned her $2,000 in ad revenue).

18. There were no apps for a year.

Not so much a secret as it is something we've largely forgotten. The App Store didn't open until a full year after the iPhone hit the market. There were 500 downloadable iPhone applications available in summer 2008; there were 2.2 million as of the start of this year.

Now the App Store is the second biggest revenue driver for Apple after the iPhone itself.

19. Let's talk about the debut, and how the engineering team got drunk during it.

Grignon and other engineers were so nervous during Jobs's unveiling that they drank Scotch throughout the whole thing. As he put it:

When the finale came -- and it worked along with everything before it -- we all just drained the flask. It was the best demo any of us had ever seen. And the rest of the day turned out to be just a [expletive] for the entire iPhone team. We just spent the entire rest of the day drinking in the city. It was just a mess, but it was great.

20. Jobs had to use multiple iPhones to make it through.

There were so many problems with the hardware, and the software was so buggy at launch, that Jobs took the stage in 2007 with about 10 of the 100 iPhones then in existence, hedging against any one of them freezing or shutting down.

21. They hard-coded the display to always show five bars of reception.

Signal strength wasn't great in the auditorium, despite bringing in a portable cell phone tower. So engineers "preprogrammed the phone's display to always show five bars of signal strength regardless of its true strength," according to Grignon.

22. Jobs foreshadowed an exodus during his presentation.

A lot of the design and engineering team was burned out and left soon after the launch. In fact, when Jobs demonstrated onstage how to delete a contact on the iPhone, he picked the name of Tony Fadell, known as the father of the iPod. Sure enough, Fadell left soon after.

23. They also had to hide the Wi-Fi for the launch.

There was a big concern that one of the "5,000 nerds in the audience" might hack the Wi-Fi, Grignon said. The solution was to "tweak the AirPort software so that it seemed to be operating in Japan instead of the United States," since "Japanese Wi-Fi uses some [different] frequencies."

24. The prototypes crashed constantly.

The prototype phones were buggy as heck, and if you tried to any of most of the things we do on a daily basis now, the phones would crash.

"Hours of trial and error had helped the iPhone team develop what engineers called 'the golden path,' a specific set of tasks, performed in a specific way and order, that made the phone look as if it worked," Grossman wrote.

25. Steve Jobs had to be seriously talked into even trying it to begin with.

Jobs was like a lot of us: It turns out he hated dealing with cell-phone companies, and dreaded the idea of having to do a development deal with one of them.

Apple had been talking about a phone since 2001, when the iPod first came out, but there were too many technical challenges. However, the big thing that eventually got Jobs excited was when laws changed and opened the wireless market.