Don't automatically think that because you're paying more you're getting more. You may only talk for three minutes, but if the service charges a ten-minute minimum, you'll be billed for it. You CAN set a designated cut-off limit and have the dispatcher instruct the phone slut to end the call after X amount of minutes.
Often, they only check availability of funds for a certain block of time and will instruct the phone slut to cut you off after that no matter how sparkling and pretty your credit may be. Phone sluts will often have more than one persona with a service.
There is typically a recorded introduction telling you to hang up if you're under 18, and disclosing what the cost per minute is and what the service offers - which can be ANYTHING.
It can be recorded fantasy, hook-up to a party line, one-on-one phone slut contact - the sky's the limit.
900 numbers: 900 numbers today don't have much cooperation from the phone companies, so they rely on third-party billing.
They can be legit businesses, or just a fly-by-night organizations hoping to make a lot of money before they get shut down for ripping off the caller. One sure bet in this category is Web900, a third-party billing service that offers flat-rate calls through various services.