Re: How many possible combinations of phone numbers are there in the US ?

Theoretically, with ten digits and ten possible values for each, there should be 10¹⁰ (10 billion) possible phone numbers. In practice, the number is a bit smaller. Because of the 1- and 0- prefixes, the first digits of the area code and local prefix cannot be one or zero. Similarly, all numbers with "area codes" or "local prefixes" of either 411 or 911 can be discounted, as can numbers with a local prefix of 555. So taking away the ones and zeros gives us 6.4 billion; less 20 million and 20 thousand for 411 and 911 put together; less 10 thousand for 555: gives us 6,379,970,000 possible US phone numbers (less any locally controlled prefixes).