Cell phones
Home phones
Company numbers
Company direct lines
Tablet SIM cards
On star etc
Elevators
Building alarm systems
Remote equipment monitoring
There are way more phone lines in use than we often think about, since many of them aren't normally used by humans. And on top of that, we have more phone lines dedicated to our personal use than simply our cell phone.
Also, many combinations of numbers can't be given out, as an easy example, you can't have a 911 area code.
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