The International Telecommunication Union (ITU) allocates call sign prefixes for radio and television stations of all types. They also form the basis for aircraft registration identifiers. These prefixes are agreed upon internationally, and are a form of country code. A call sign can be any number of letters and numerals but each country must only use call signs that begin with the characters allocated for use in that country.

Maritime identification digits are used by ship-to-shore radiocommunication facilities to identify their home country or base area in Digital Selective Calling and Automatic Identification System messages, as part of their Maritime Mobile Service Identities. The International Telecommunication Union facilitates the assignment of MIDs to countries. This is a comprehensive list of MIDs that each country around the world uses. Note that not all countries have MIDs; such countries are typically landlocked, with no access to international waters. Sorting MID assignments in numerical order reveals a regional structure, with the first digit 2 assigned to Europe, 3 to North America and the Caribbean Sea, 4 to Asia (not the southeast), 5 to the Pacific and Eastern Indian Oceans and Southeast Asia, 6 to Africa, the Atlantic and Western Indian Oceans, and 7 to South America.

The radiocommunication division of the International Telecommunication Union uses the following letter codes to identify its member countries. Eight countries are assigned single-letter codes, while the rest have codes three letters in length.