Telephone numbering
Telephone numbering is how phones are addressed on telephone networks ranging from the old-fashioned wire-based "Plain Old Telephone Service" (POTS) to mobile networks, voice-over-IP (VOIP), and other services.
In the very very old days (probably before just about everybody living now was born), phones had no dial and you had to talk to an operator to be connected, even for local calls. The phone numbers were assigned at the local-exchange level and might be as small as one digit (as in the child's rhyme, "Hello operator, get me number nine"). Then direct-dial was added for local calls (still with variable-length numbers), but you needed an operator for long distance. Eventually, phone numbers in the United States were standardized at seven digits for the local number (a three-digit exchange and four other digits) and a three-digit area code before it, so that calls could be direct-dialed across the country. Other countries did their own standardization in different ways. (For a time, U.S. phone numbers were usually expressed with letters in the exchange part, which translated to numbers in a scheme shown on the phone dials, but eventually all-digit numbers were used, though the letters persisted for marketing uses where businesses got numbers which spelled something meaningful.)
Eventually, an international system was adopted consisting of a country code followed by the country-specific phone number (including any area or city codes that country might use). The standard international way to write a phone number (and the way it usually appears when phone numbers are used as an element in a file format) is with the "+" character followed by the country code and the number, with optional spaces as separators between parts of the number. Since the country code for the U.S. is 1, numbers end up like "+1 212 555 1212". It happens that "1" is usually the number you need to dial before a long-distance call (though this varies from area to area), so (other than the + sign) that closely represents what you actually dial. For calls to or from other countries, however, an international prefix needs to be dialed (in place of the + sign), such as "011" for calls from the U.S. to other countries. (However, in some mobile networks, you can actually enter a "+" preceding an international number to place the call, so you can use international-standard-written numbers "as is".)
Within a country, there are usually some more "parochial" ways to write numbers that omit the country code (and sometimes the area code as well if it's aimed at a local audience, though these days mobile phone users often keep their number when they move, so the area code is no longer a reliable indication of location). For instance, U.S. numbers can be written as "(212) 555-1212" or "212-555-1212" or "555-1212". These can be ambiguous if you don't know the area or country the number is in.
U.S. letter-to-number correspondence
| Number | Letters |
|---|---|
| 1 | (no letters) |
| 2 | A B C |
| 3 | D E F |
| 4 | G H I |
| 5 | J K L |
| 6 | M N O |
| 7 | P R S |
| 8 | T U V |
| 9 | W X Y |
| 0 | (no letters; often labeled 'Operator') |
1 and 0 had no letters associated with them. Dialing 0 by itself reached an operator. Q and Z were not assigned any number; in modern cell phones where text input by number keys is needed, Q is usually added to 7 and Z to 9.
