In some cases, telephone spoofers use telephone numbers that seem quite plausible at first glance, but are not valid. Telephone numbers must comply with a certain set of rules, e.g. regarding length and structure. Invalid telephone numbers do not comply with these rules. If you try to call back such a number, you usually receive an announcement that it is an invalid number.
The somewhat more detailed explanation is as follows:
An invalid phone number is a number that does not comply with the international standards and criteria for telephone numbers. Telephone numbers are structured according to an internationally valid system, the E.164 numbering plan of the International Telecommunication Union (ITU). For a phone number to be valid, it must meet the following criteria:
- Country code: Each country has a unique country code, which usually starts with a plus sign (+) followed by a number, e.g., +49 for Germany or +1 for the United States. A valid phone number must include the correct country code.
- Area code: This is an additional sequence of digits that follows the country code and identifies the geographical area within a country. The area code varies depending on the country and region. A valid phone number must contain the appropriate area code.
- Local number: The local number is the part of the telephone number that follows the area code and usually consists of a specific number of digits, depending on the rules of the respective country. A valid phone number must contain a correct local number.
- Length: Phone numbers have a set length that varies depending on the country. A valid phone number must be within this specified length.
- Format: Depending on the country and region, certain formatting rules may apply, such as the use of hyphens or spaces to separate digits. A valid phone number should comply with these formatting rules.
If a phone number does not meet one or more of these criteria, it is considered invalid. An invalid phone number, for example, could have an incorrect country code, an incorrect area code, an incorrect local number, or an incorrect length. An invalid phone number may also result from a typo or incomplete input.
For further information on the subject of telephone numbers and international standards, you can refer to the following sources:
- International Telecommunication Union (ITU) – The ITU is the United Nations organization responsible for information technology and communication. You can visit their official website to obtain information about the E.164 numbering plan and other international telephony standards: https://www.itu.int/
- Wikipedia – Wikipedia provides extensive articles on telephone numbers, including information on country codes, area codes, and various national numbering plans: https://en.wikipedia.org/wiki/Telephone_number
- Country-specific regulatory authorities – In many countries, there are regulatory authorities responsible for telecommunication services. These authorities often publish information on national numbering plans and telephony standards. For example, the Federal Network Agency in Germany: https://www.bundesnetzagentur.de/
- Telecommunication providers – Your local telecommunication provider can also provide you with information and resources on the topic of telephone numbers. You can contact their customer service or visit their website to obtain more information.
These sources should give you a good overview of the subject of telephone numbers and international standards.