London calling

"London Calling" is a song by The Clash;  the title alluding to the "This is London calling..." identification used for BBC World Service broadcasts in World War II.

The BT Tower in London was opened in 1966 by the General Post Office (which is why some people call the building the Post Office Tower). The communications part of the GPO became British Telecom in 1981 which in turn was rebranded as BT in 1992.