Gloria Leonard, a pornographic entertainer turned magazine editor, launched the first phone sex line on January 1, 1977. Ms. Leonard, at that time, worked as the editor of High Society which was a popular adult entertainment magazine. She is credited for initiating the idea of a phone sex line to promote her hardcore sex magazine to its target readers. She started by recording her own voice to read the erotic contents of High Society magazine. 

The original model was quite naive and innocent. It features the recorded voice of a female who was actually reading a few sexy and erotic lines from the magazine. Callers paid on a per-minute basis just to listen to what the voice is saying which is, by all means, all about sex. 

Its phenomenal success led to a bolder concept because if men were willing to pay money just to hear a woman talking about sex, there was so much greater opportunity in getting actual females to speak live to the male callers. From here, it was realized that “sex sells” like hot pancakes even via telephone lines; and this was how modern phone sex was born to eventually become a thriving business. 

From another resource, it is indicated that the first invention of the telephone marked the initial steps towards developing the phone sex model which has become extremely popular these days. It just continued to evolve over time thus giving birth to various forms of erotic chat lines and dating lines.