It's absolutely true. All US numbers are 555-xxxx. You can see this in any US movie or TV show, which we know to be very accurate.
Similarly, any time an American car has a fender-bender, or at least one of its wheels leaves the ground, it explodes in a massive fireball.
That's what's really holding me back from getting an American car. Just fearing that somebody might hit my car and then it's over.
Independent of Hollywood, some American cars just might do that. Maybe not in such an impressive manner, but I've been through so many Dodge transmissions and Ford's reputation here is even worse.
joking aside, the 5M figure probably came from targeting like this, such as choosing a few area codes with high tech populations and testing the ~10M phone numbers for each area