A call girl or female escort is a prostitute who (unlike a street walker) does not display her profession to the general public, nor does she usually work in an institution like a brothel, although she may be employed by an escort agency. The client must make an appointment, usually by calling a telephone number. Call girls have traditionally had a number of routes available to advertise their services, including classified advertisements in magazines and latterly via the Internet.
https://en.wikipedia.org/wiki/Call_girl