Why haven’t we run out of cellphone numbers?

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The US uses a 10-digit system. Each digit can be any of the 10 numerals in our number system; that means there are 10x10x10x10x10… or 10^10 possible numbers in the US alone. That makes for 10 billion phone numbers possible, for a population of about 350 million. Each person can get over 25 unique phone numbers dedicated to themselves before we run out.

Other countries use different numbers of digits, so there are even more possibilities.

There is also an extra digit or set of digits, known as the country code, that tells the phone which country’s phone system to reach out to. So that gives even more possibilities, as different countries can use the same phone number and rely on the country code to tell the phones apart.

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