Turkish Airlines is the national airline with the Turkish flag. The business is headquartered in the General Management Building of Turkish Airlines on the Ataturk Airport grounds in Istanbul. It operates scheduled services at 40 domestic airports and 197 international airports in Europe, Asia, Africa and the Americas as of August 2013. Turkish Airlines is flying to 103 countries. The carrier's destination network from one hub is the biggest in the world. With 237 destinations, the airline is considered by number of destinations to be the fourth largest carrier in the world. Since Ataturk International Airport is the main Turkish carrier, the airline has secondary hubs at Esenboga International Airport, Sabiha Glen International Airport and Adnan Menderes Airport. As of January 2013, 15,978 workers were hired by the airline. In April 2010, 'TURKISH' replaced 'TURKAIR' as the current call sign for Turkish Airlines.