I am certainly no expert on this subject, but I know a couple of things which may be of interest to those who like to peer under rocks.
Types of Lines:
Toll Numbers: 900 numbers and so on, these lines are set up to charge anyone who dials them. They ought to show up on your phone bill with some relatively innocuous telecom-sounding name attached, but who is really gonna be fooled by that?
Long-Distance Numbers: Frequently these point to somewhere in the Caribbean, and are configured to charge the caller just like a 900 number. These will show up on your phone bill as is, and if you aren't the only one in the household who looks at the bill, they may generate some disconcerting questions. (Like, "Who the hell's been calling Trinidad?")
Toll-Free Numbers: These have an introductory message welcoming you to the site and asking for your credit card number. The actual credit card part is handled by another agency, so if there is an issue with the card you will likely find yourself forwarded to them instead of the promised "hot vixens". The charges you incur will show up on the credit card bill, again as a bland, this-fools-no-one company name.
And here's a fun fact: if you lose track of time and the charges exceed the limit on the card, a bill went be sent for the remainder. But it won't be sent to the owner of the card; it will be sent to the person whose name is on the phone bill for the number you dialed from. Imagine if you will your roommate or parent getting a bill for two hundred dollars from a company they've never heard of. Bewildered, they call the company to find out what they're being charged for, and...well, I'll leave the rest to your imagination.
Types of Services:
Recordings: A menu is provided of recorded messages in which different allegedly erotic scenarios are described. Each message lasts roughly ten minutes, presumably as that provides sufficient time to do yer business.
Party Lines: Like a chat room, with actual chat! There is almost always someone on the line who is an employee of the company, looking for someone she can keep on the line for a long time. They will invite this person into a private "room" and talk the night away with them while the meter runs on and on...
Operators: There tend to be two stereotypes of operators: "porn star" and "obese housewife in a muumuu". I've known three people who worked at phone sex lines and they were very lovely women. These are just ordinary folks for the most part, trying to pay the bills just like everyone else. They will stick to a prepared script if at all possible, and may not really be paying attention to what you're saying. If you're into the idea of sex with someone who picks up a magazine in the middle of your performance and starts reading, this may be for you. (See Confessions of a Former Phone Sex Operator for the other side of the looking glass...)
Classifieds: Phone mailboxes in which people who want phone sex leave their names, numbers, a time you can call, and a short message. You will of course have to pay by the minute to listen to this information.