What is the callsign of an airplane transporting the US president?

The presidential air transport fleet consists of two specially configured Boeing 747-200B's — tail numbers 28000 and 29000 — with the Air Force designation VC-25. When the president is aboard either aircraft or any Air Force aircraft, the radio call sign is "Air Force One." Presidential air transport began in 1944 when a VC-54, nicknamed the "Sacred Cow," was put into service for Presidents Franklin D. Roosevelt. President Roosevelt’s successor, President Harry S. Truman, used the aircraft extensively during the first 27 months of his administration. On July 26, 1947, President Truman signed the National Security Act of 1947 on board the Sacred Cow. This act established the US Air Force as an independent service, making the Sacred Cow the “birthplace” of the US Air Force.

Source: Official United States Air Force Website