Telephone service in the United States has now gone to a 10-digit phone number system. The parts of a telephone number in the US are the 3-digit area code, the 3-digit exchange then the 4-digit personal number. The area code and exchange route the call to the correct country, state, city, and possibly to an area of that city if the city has more than one area code.
A "1" is added to indicate "long distance".
I don't know much about overseas phone numbers except that each has a country code. The US also has a country code if dialing from outside the US.
It varies from country to country. The country code can be one digit (+1 = US, Canada, etc.; +7 = Russia & Kazakhstan), two digits (+20 = Egypt; +27 = South Africa; +30 = Greece; etc.), or three digits (most countries). The national number, including the area code where applicable, can be anywhere from 4 to about 12 digits, but the total of the country code and national number cannot be more than 15 digits. In some countries, the number of digits is fixed; for example, all numbers in the US and Canada are a total of 11 digits: country code +1 (1 digit) + area code (3 digits) + local number (7 digits). In other countries, the number of digits may vary within the country, or even within a particular city or town.
In the US and Canada, telephone numbers have 10 digits, including the area code. Other countries use phone numbers with different numbers of digits.
Phone numbers are seven digits long in some countries but differing lengths in others. It purely depends upon what number of phone numbers the country needs .
Telephone numbers throughout Russia are uniformly 3 digits for the area code plus 7 digits for the local number.
10 digits are numbers in the billions.
the area code
the area code
Any variation of 10 digits.
Because then they are more complicated so there can be more phone numbers. Imagine if phone numbers were 3 numbers long! We would have run out a long time ago.
there are 899 whole numbers that have three digits.
Assuming you can use the values listed, and the phone number is 7 digits (e.g. XXX-XXXX), then you can select 3 numbers for the first digit (0 is exempt) then 4 selections for the rest of the digits. So, 3*4*4*4*4*4*4 = 12288 phone numbers.