Area Code 312 / +1312
The prefix 312 is the area code for Illinois. And Illinois is located in United States of America. If you are outside France and you want to call a person in Illinois, in addition to the area code, you need the country code for the country you wish to call. The country code for France is +1, so if you are in France and you want to call a person in Illinois, you have to prefix that person’s phone number with +1 312.
The zero in front of the area code is omitted in this case.
The plus sign at the beginning of the phone number can usually be used in this format. However, it’s more common to replace the plus sign with a sequence of numbers that alerts the telephone network that you want to dial a telephone number in another country. The ITU recommends using 00, which is also used in many countries, including all European countries.
As an alternative to +1 312, which you have to place in front of the telephone number of a person in Illinois in order to call them from France, you can also use 001 312.
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