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FALSEHOODS.md
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+# Falsehoods Programmers Believe About Phone Numbers | ||
+ | ||
+... and tips on how to use libphonenumber. | ||
+ | ||
+Given how ubiquitous phone numbers are and how long they've been around, it's | ||
+surprising how many false assumptions programmers continue to make about them. | ||
+ | ||
+1. **Phone numbers that are valid today will always be valid. Phone numbers of | ||
+ a certain type today (e.g., mobile) will never be reassigned to another | ||
+ type.** | ||
+ | ||
+ A phone number which connects today may be disconnected tomorrow. A number | ||
+ which is free to call today may cost money to call tomorrow. The phone | ||
+ company may decide to expand the range of available phone numbers by | ||
+ inserting a digit into an existing number. | ||
+ | ||
+ **Tip:** Don’t store properties for a phone number such as validity or | ||
+ type. Check this information again from the library when you need it. | ||
+ | ||
+1. **A phone number uniquely identifies an individual** | ||
+ | ||
+ It wasn't even that long ago that mobile phones didn't exist, and it was | ||
+ common for an entire household to share one fixed-line telephone number. In | ||
+ some parts of the world, this is still true, and relatives (or even friends) | ||
+ share a single phone number. | ||
+ | ||
+1. **An individual has only one phone number** | ||
+ | ||
+ Obviously, this isn't necessarily true. | ||
+ | ||
+1. **Phone numbers cannot be re-used** | ||
+ | ||
+ Old phone numbers are recycled and get reassigned to other people. | ||
+ | ||
+1. **Each country calling code corresponds to exactly one country** | ||
+ | ||
+ The USA, Canada, and several Caribbean islands share the country calling | ||
+ code +1. Russia and Kazakhstan share +7. These are not the only examples! | ||
+ | ||
+1. **Each country has only one country calling code** | ||
+ | ||
+ As of this present moment (in Mar. 2016), phones in the disputed territory | ||
+ and partially recognised state of Kosovo may be reached by dialing the | ||
+ country calling code for Serbia (+381), Slovenia (+386), or Monaco (+377), | ||
+ depending on where and when one obtained the number. | ||
+ | ||
+ **Tip:** Use the phone widget to encourage users to enter their phone number | ||
+ in an international format such that we can understand it. | ||
+ | ||
+1. **A phone number is dialable from anywhere** | ||
+ | ||
+ Some numbers can only be dialed within the country. Some can only be dialled | ||
+ from within a subset of countries, such as the international 00800 numbers. | ||
+ Some may be dialable only if the caller is a subscriber to a particular | ||
+ telecom company. | ||
+ | ||
+1. **There are only two ways to dial a phone number: domestically and from | ||
+ overseas** | ||
+ | ||
+ Some numbers may need different prefixes depending on: the carrier you are | ||
+ using; what device you are dialling from/to; whether you are inside or | ||
+ outside a particular geographical region. | ||
+ | ||
+ Examples: | ||
+ * In Brazil, to dial numbers internally but across a certain geographical | ||
+ boundary, a carrier code must be explicitly dialed to say which carrier | ||
+ you will use to pay for the call. | ||
+ * In Nepal, the leading zero in national format is omitted depending on | ||
+ whether the originating phone is mobile or fixed-line. | ||
+ * In New Zealand, you need to dial the area-code (e.g. 03) even if the | ||
+ number is within the same area-code region as you are, unless it is | ||
+ "close" (something approximating city/district boundaries), in which | ||
+ case it shouldn’t be dialled. | ||
+ | ||
+ **Tip:** Use formatForMobileDialling to get the number a user should | ||
+ actually dial on their mobile phone. | ||
+ | ||
+1. **To make a number dialable, you only need to change the prefix** | ||
+ | ||
+ In Argentina, to dial a mobile number domestically, the digits "15" need to | ||
+ be inserted *after* the area code but *before* the local number, and the "9" | ||
+ after the country code (54) needs to be removed. This transforms +54 9 2982 | ||
+ 123456 into 02982 15 123456. | ||
+ | ||
+1. **No prefix of a valid phone number can be a valid phone number** | ||
+ | ||
+ In some countries, it's possible to connect to a different endpoint by | ||
+ dialing more digits after a number. So "12345678" may not reach the same | ||
+ person as dialing "123456". | ||
+ | ||
+1. **An invalid number will not reach an endpoint** | ||
+ | ||
+ In some countries, or on some phones, extra digits are thrown away. Hence, | ||
+ 1-800-MICROSOFT is an invalid number - but it still connects to Microsoft, | ||
+ since any later digits are ignored. Numbers such as "911" can be reached by | ||
+ dialling "911 123" in some countries: but not in others. | ||
+ | ||
+ In other countries, invalid numbers may be "fixed" by a carrier, e.g., | ||
+ adding a mobile token if they know it is a mobile number, such that it | ||
+ connects. | ||
+ | ||
+1. **All valid phone numbers follow the ITU specifications** | ||
+ | ||
+ ITU says things like "national numbers can not be longer than sixteen | ||
+ digits" but valid numbers in Germany have been assigned that are longer than | ||
+ this. | ||
+ | ||
+1. **All valid phone numbers belong to a country** | ||
+ | ||
+ There are many "country calling codes" issued to non-geographical entities, | ||
+ such as "800" or satellite services. | ||
+ | ||
+1. **Phone numbers contain only digits** | ||
+ | ||
+ In Israel, certain advertising numbers start with a `*`. | ||
+ | ||
+1. **Phone numbers are always written in ASCII** | ||
+ | ||
+ In Egypt, it is common for phone numbers to be written in native digits. | ||
+ |
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