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నంబరింగ్ ప్లాన్లో అనేక రకాల డయల్ ప్లాన్లు ఉండవచ్చు మరియు ఇవి తరచుగా స్థానిక టెలిఫోన్ ఆపరేటింగ్ సంస్థ యొక్క నెట్వర్క్ నిర్మాణంపై ఆధారపడి ఉంటాయి.
== మూలాలు ==
The North American Numbering Plan (NANP) divided the North American service territories into numbering plan areas (NPAs), and assigned to each NPA a unique numerical prefix, the numbering plan area code, which became known in short-form as area code. The area code is prefixed to each telephone number issued in its service area.
National telecommunication authorities use various formats and dialing rules for area codes. The size of area code prefixes may either be fixed or variable. Area codes in the NANP have three digits, while two digits are used in Brazil, one digit in Australia and New Zealand. Variable-length formats exist in multiple countries including: Argentina, Austria (1 to 4), Germany (2 to 5 digits), Japan (1 to 5), Mexico (2 or 3 digits), Peru (1 or 2), Syria (1 or 2) and the United Kingdom. In addition to digit count, the format may be restricted to certain digit patterns. For example, the NANP had at times specific restrictions on the range of digits for the three positions, and required assignment to geographical areas avoiding nearby areas receiving similar area codes to avoid confusion and misdialing.
Some countries, such as Uruguay, have merged variable-length area codes and telephone numbers into fixed-length numbers that must always be dialed independently of location. In such administrations, the area code is not distinguished formally in the telephone number.
In the UK, area codes were first known as subscriber trunk dialling (STD) codes. Depending on local dialing plans, they are often necessary only when dialed from outside the code area or from mobile phones. In North America ten-digit dialing is required in areas with overlay plans.
The strict correlation of a telephone to a geographical area has been broken by technical advances, such as local number portability and Voice over IP service.
When dialing a telephone number, the area code may be preceded by a trunk prefix (national access code), the international access code and country code.
Area codes are often quoted by including the national access code. For example, a number in London may be listed as 020 7946 0321. Users must correctly interpret 020 as the code for London. If they call from another station within London, they may merely dial 7946 0321, or if dialing from another country, the initial 0 should be omitted after the country code.
== మూలాలు ==
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