Green’s Dictionary of Slang

call-girl n.

also call mom
[SE call, to make a telephone call + girl n.1 (1)]

1. [late 19C+] a woman who works in a brothel.

2. [1910s+] (orig. US) a prostitute who advertises her services through an agency, through the (print) media, in telephone kiosks etc and visits a client in his own home or hotel room; also attrib.