The Hotline to Hell (Jigoku Tsūshin), known in the English version as the Hell Correspondence, is the medium of contacting Ai Enma, known as the Hell Girl (Jigoku Shoujo) for requesting her service, i.e. to get one's tormentor ferried straight to hell. Once the Hotline to Hell is accessed and a request is sent, Ai decides whether she would accept the request or not based on a lot of judgemental points. If she does accept the request, she appears to the person who requested her service and temporarily takes the client to her world, where she makes a contract with the client before actually providing her service.
The Hotline to Hell has actually existed throughout the last four centuries, in other shapes and forms. An old man, Fukumoto, who accessed the Hotline to Hell in his youth, remembered having to find Ai through a personals ad in a newspaper. Only at midnight could those with grudges see the advertisement, and, after having read the address contained in the ad, they would have to write the name of their tormentor on a piece of paper and post it to the Hotline to Hell. In the days before newspapers, those who sought revenge would have to write the name of their enemy on an ema (small wooden plaques on which worshippers write their prayers or wishes) and leave it at a shrine at midnight. This was known as the Jigoku Ema. However, as time and technology has advanced, the Hell Girl has changed her methods of keeping contact, and the Hotline to Hell has evolved into an Internet phenomenon.
Currently, it exists as a website where one can enter the name of their tormentor and send it to Ai in hope that she will accept their request and provide her service by taking their target to hell. The website is very simple, with the only contents being a text box for entering the name of the tormentor, a send button below it for sending the name typed in, and a text over the text box saying "We will deliver vengeance, on your behalf".
It has been shown that Ai also runs an SMS service in conjunction to her website. She normally uses the SMS service to send a confirmation note to the client. When the text message is received, the screen glows red. The content of the SMS is the message "Your message has been received". This is mainly used for giving the client a reassurance that she, along with her service, is in fact real.